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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 9, Issue 9, September 2018, pp. 945–967, Article ID: IJMET_09_09_104

Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=9&IType=9

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

HEAT TRANSFER IMPROVEMENT IN A SOLAR

AIR HEATER BY USING DIFFERENT TYPES OF

ARTIFICIAL ROUGHNESS: A REVIEW

J L Bhagoria

Professor, Department of Mechanical Engineering,

Maulana Azad National Institute of Technology, Bhopal, India

Yogesh Agrawal

Research Scholar, Department of Mechanical Engineering,

Maulana Azad National Institute of Technology, Bhopal, India

ABSTRACT

Heat transfer in the field of thermal engineering plays an important role. Artificial

roughness can be applied in many types of roughness geometry element which can be

two and three dimensional ribs, sand grain, combination of rib and groove rib,

grooves, compounding ribs, wire fixation, transverse or inclined type of ribs, dimple /

protrusion formation type of geometry and expanded wire mesh fixation. An

artificially roughened surface of roughness plate is believed an efficient for the

increment of heat transfer. For the last thirty years, the use of artificial roughness

elements in air heater had been a good topic for research. Multiple investigators have

carried out so many numerical and observational works. Various roughnesses

geometric in literature have been reported by investigators to study the high

temperature transfer and friction characteristics of an artificially roughened duct of

solar air heaters. For increasing the heat transfer among a heat transfer technique,

artificial roughness is one of the most valuable techniques. Many investigators found

about the application of artificial roughness in the form of ribs. The literature of the

roughened heat transfer system by using different kinds of artificial roughness is

covered by this paper. To review the performance of various rib geometry employed

for developing artificial roughness, this attempt is formed. Based on correlations

observed by different researchers for friction factor, and heat transfer coefficient,

Improvement of heat transfer coefficient and reduction of friction factor is considered.

It has also reviewed and presented the thermo-hydraulic performance of different

geometry on its absorber surface of solar air heater duct has been compared.

Keywords: Artificial roughness, Heat Transfer, Pressure drop, solar air heater, solar

energy.

Heat Transfer Improvement in a Solar Air Heater by Using Different Types of Artificial

Roughness: A Review


Cite this Article: J L Bhagoria and Yogesh Agrawal, Heat Transfer Improvement in a

Solar Air Heater by Using Different Types of Artificial Roughness: A Review,

International Journal of Mechanical Engineering and Technology, 9(9), 2018, pp.

945–967.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=9

NOMENCLATURE Physical Parameters

Ac surface area of absorber plate, mm2

B half length of V-rib element, mm

Cp specific heat of air, J/kg K

Dh equivalent or hydraulic diameter of duct, mm

D diameter of dimple or protrusion, mm

E roughness height, mm

G groove position, mm

H height/depth of duct , mm

h heat transfer coefficient, W/m2K

I intensity of solar radiation, W/m2

K thermal conductivity of air, W/mK

L length of test section of duct, mm

L longway length of mesh, mm

M mass flow rate, kg/s

P pitch, mm

Qu useful heat gain,W

qu useful heat flux, W/m2

S shortway length of mesh, mm

Tpm mean plate temperature, K

Tam mean air temperature, K

Ti fluid inlet temperature, K

Ta ambient temperature, K

To fluid outlet temperature, K

UL overall heat loss coefficient, W/m2K

V velocity of air in the duct, m/s

W width of duct, mm

W width of rib, mm

∆p pressure drop, Pa

Geometrical dimensionless parameters

B/s relative roughness length

d/D relative print diameter

DD relative discrete distance

d/w relative gap position

d/α relative gap distance

e/Dhh relative roughness height

FR collector heat removal factor

fr friction factor for rough surface

Gd/Lv relative gap distance

g/e relative gap width

g/p relative groove position

HD/HD relative baffle height

l/e relative long way length of mesh

l/s relative length of metal grid

Ng number of gaps

Nu Nusselt number

PB/HB relative baffle pitch

p/e relative roughness pitch

J L Bhagoria and Yogesh Agrawal


p’/p relative staggered rib pitch

Pr prandtl number

Re Reynold number

s/e relative short way length of mesh

s’/s relative roughness segment ratio

W/H duct aspect ratio

WD/HD channel aspect ratio

WD/WB relative baffle width

W/w relative roughness width

w/e staggered rib length to rib height ratio

α/90 relative angle of attack

Greek Symbols

(τα)e Effective transmittance absorptance

product

ɸ wedge angle/chamfer angle, degree

Α angle of attack, degree

Δ transition sublayer thickness, mm

ƞth thermal efficiency

Ρ density of air, kg/m3

Acronym

CFD Computational Fluid Dynamics

THPP Thermo-Hydraulic Performance Parameter

1. INTRODUCTION

Energy in various structures has assumed a decisive part in Globally for Economic advance

and industrialization. The Non-renewable energy resources are fixed (in the historic period of

energy shortage). Therefore, more consideration must be paid to upgrade and use the

sustainable power source assets. Sun is a definitive wellspring of vitality. All the patterns of

energy in the universe as we know it are solar in origin. The best points of interest of sun

based energy as contrasted and different strains of energy are that it is spotless and can be

provided without ecological contamination. [1] It is an effective process of enhancing thermal

efficieny of conventional solar air heaters is by implementing roughness on the bottom of an

absorber in terms of ribs, grooves, baffles, winglets, twisted tapes etc. To enhance the

performance of solar air heater in terms of thermo hydraulic can be improved by a passive

technique of heat transfer that is called artificial roughness. The device, which absorbs

incoming solar radiations and converts it into thermal energy, is called a solar air heater at the

absorbing surface. Many practices have been prepared by multiple investigators in their

research work for gaining the heat transfer increment through these solar air heaters by

applying various roughness elements on the surface of the absorber plate. Many researchers

developed friction factor correlation and heat transfer coefficient for roughened duct of solar

air heater have been studied in this paper. This paper summarizes all these efforts of attempt

and to achieve the conclusion about past experimentation works. This is a chance for

investigators to create the new parameters with materials and methods to get the satisfied

result of improvement of heat transfer with the decrement of the friction power provision. [2].

2. THEORY

2.1. SOLAR ENERGY CLASSIFICATION

Solar Energy is the best effectively accessible wellsprings of progression. This is one of the

non-conventional sources of energy because it is pollution free, hence aids in lessening the

greenhouse impact.

The sun's energy can be classified in two forms:


Roughness: A Review


2.1.1. Heat or Thermal Energy (ii) Light Energy

Solar thermal methodology application the solar heat energy to heat substances like air or

water for use like pool heating, space heating and water heating for residencies and

commercial. The solar heat can be helpful for generating electricity on a big scale by

transforming the solar energy into mechanical energy.

2.2. SOLAR THERMAL COLLECTORS AND ITS TYPES

A device, which is used to collect solar radiation and displaces the energy to a fluid flowing in

contact with it is called solar thermal collector. The use of solar energy needs solar collectors.

Solar collectors can be either focusing or non-concentrating. In the non – concentrating form,

the area that intercepts the solar radiation that is collector area is the same as the area

absorbing the radiation that is absorber area. In this method, light is absorbed by the whole

solar panel. Concentrating collectors are having a larger interceptor rather than absorber as

shown in Fig.1 [3]. Multiple forms of solar thermal collectors are:

2.2.1. FLAT PLATE TYPE OF SOLAR COLLECTOR (NON CONCENTRATING TYPE)

As shown in the Fig. 2 for space, heating, drying and similar industrial application demanding

heated air at low to moderate temperature by applied flat plate solar air heaters. A function of

many designs and operating parameters are used to calculate the thermal efficiency of a solar

air heater (collector) .The Convective heat transfer coefficient is one of the key parameters

between the absorber plate and air flowing through the collector duct. They include of (1) a

transparent cover that decreases heat losses, (2) a heat insulating backing, (3) A heat transport

fluid it (air, antifreeze or water) to reduce heat from the absorber, and (4) A dark flat plate

observer. It can be found directly or by using a heat exchange as per shown in Fig.3.



2.2.2. CONCENTRATING (FOCUSING) TYPE SOLAR COLLECTOR

As shown in Fig.4 Focusing or Concentrating Collectors intercept direct radiation over a huge

area and concentrate it onto a small absorber area. To get more efficient high temperature than

flat plate collector can be provided by these collectors because the absorption surface area is

much smaller. Hence diffused sky radiation cannot be concentrated onto the absorber. To

constantly orient the collectors towards the sun and to keep the absorber at the point of focus

mechanical equipment is required by the most concentrating collectors as shown in Fig.5 & 6.


Roughness: A Review


2.3. THE PERFORMANCE IMPROVEMENT OF SOLAR AIR HEATER

a. Increment of Intensity of solar radiations occurrence upon the solar collector

b. Decrement of thermal losses by;

i. Application of multiple glass covers

ii. Application of substitute medium or Vacuum in space;

c. Selective coating on the absorber plate by:

i. Application of two pass system

d. Enhancement of heat transfer from absorber plate to the air by;

i. Application Enlarged surfaces

ii. Packed bed solar heaters

iii. (iii)Application of artificial roughness on absorber plate

2.4. BASICS OF ARTIFICIAL ROUGHNESS

To improve thermo hydraulic performance of solar air heater a passive heat transfer

improvement technique is developed which is called artificial roughness is developed to

improve forced convective heat transfer, which needs flow at the heat-transferring surface to

be turbulent. Hence, Turbulence has to come from the fan or blower for creating energy and

to flow air through the duct, excessive power is needed. Therefore, it is mandatory that the

turbulence must be developed only in the area very near to the heat-transferring surface, to be

lessened power requirement. It can be done by making the height of the roughness element to

be short as compared with the duct dimension. To significantly improve the heat transfer

coefficient with minimum pressure loss penalty, artificial roughness on heat transferring

surface of a symmetrically high aspect ratio rectangular duct modeled as solar air heater ducts

has been shown.

Karwa et al. [13] Examined Experimentally and studied of develop heat transfer and

friction in the transverse, inclined, V-continuous and V-discrete pattern by utilizing two

identical parallel ducts, one with the roughened absorber plate and the other with the smooth

one in a symmetrically heated rectangular duct with ribs on the heated wall. The roughened

elements had the rib chamfer angle was fixed at 150 while a relative roughness pitch (p/e) of

4.58 and 7.09.This study demonstrated a significant enhancement in the Nusselt number (50-

120%) due to the enhancement in thermal efficiency (10-40%) over solar air heaters with

smooth absorber plates.

Bopche et al. [23] Examined the heat transfer coefficient and friction factor on the

absorber surface of an air heater duct for specially prepared inverted U-shaped turbulators.



The roughened wall was uniformly heated and the remaining three walls were insulated. The

experiments surrounded the Reynolds number range from 3800 to 18,000, e/Dh varied from

0.0186 to 0.03986 and p/e varied from 6.67 to 57.14, the angle of attack of the flow on

turbulators α=900 kept constant during the whole experimentation. As compared to the

smooth duct, the turbulator roughened duct enhances the heat transfer and friction factor of

2.82 and 3.72 times respectively.

Kumar et al. [24] researched the thermo-physical performance of solar air heaters with

discrete W-shaped ribs and the parameters W/H=8;e/Dh=0.0168-0.0338;p/e=10;Re=3000-

15,000;α=30-750 on the absorber plate .Also developed the correlations for a Nusselt number

and friction factor using the range of parameters selected. They found that the increase in

Nusselt is 2.16 and friction factor is 2.75 at e/Dh=0.0338 and α=600 as compared to smooth

solar air heater.

Lanjewar et al.[29] examined experimental investigation with W-shaped ribs arranged at

an inclination with respect to flow direction on its underside on one broad wall for heat

transfer and friction factor characteristics of rectangular duct roughened. Range of parameters

was used are W/H=8, p/e=10, e/Dh=0.018-0.03375, Re=14000 and α=30-750. Max.

Enhancement of a Nuselt number and friction factor was found to be respectively 2.36 and

2.01 times that of the smooth duct as a result of providing artificial roughness for an angle of

attack of 600.

Gawande et al. [36] researched methodologies of numerical modeling and simulations for

thermal achievement optimization with 200 angled ribs were imitated used an algorithm

created in MATLAB to assume the most favorable set of design and performing set of

parameters. The simulation is done using correlations developed using second order

polynomial. The effect of heat flux, velocity, variation in the width of the duct, mass flow rate

and the no. of glass covers, on thermal & effective efficiencies of roughened solar air heater

are described in the form of plots.

2.5. HEAT TRANSFER IMPROVEMENT THROUGH ARTIFICIAL

ROUGHNESS IN SOLAR AIR HEATER

Most extreme heat transfer coefficient is utilized as a part of sunlight based air radiators using

the artificial roughness up to laminar sub-layer .The nearness of rib may upgrade heat transfer

in view of intrusion of the viscous sub-layer, which yields streamed turbulence, partition, and

reattachment prompting a higher heat transfer coefficient. The ribs are given just on the

heated divider. The other three dividers are smooth (without ribs) and shield. It is along these

lines alluring that the turbulence must be made just in the area near the heat exchanging

surface, i.e., In the laminar sub-layer just where the heat trade happens and the stream ought

not be unduly exasperates to stay away from over the top grinding misfortunes. In spite of the

fact that there are a few parameters that describe the course of action and state of the

harshness, the roughness element height (e) and pitch (p) are the most critical parameters.

These parameters are typically determined as far as dimensionless parameters, to be specific,

relative roughness height (e/Dh) and the relative roughness pitch (p/e).

2.6. PERFORMANCE ANALYSIS OF SOLAR AIR HEATER

It is required to analyze thermal and hydraulic performance; the design of solar air heater

should be efficient. It deals with a heat transfer process within the collector and hydraulic

performance deals with pressure drop in the duct. Both are given below:


Roughness: A Review


2.6.1. THERMAL PERFORMANCE

The useful energy gain (Qu) across the test section was calculated from the temperature to gain

across the test section and air mass flow rate (m) by using equation (1)

Qu=mCp(To-Ti) (1)

The convection heat transfer coefficient (h) from the absorber plate to air was measured

from mean temperature of the plate (Tpm) and air (Tam) using equation (2). It can be enhanced

by implementing artificial roughness on the surface of the absorber plate.

Qu =hAc (Tpm-Tam) (2)

It can be described in non-dimensional forms by using the following relationship of the

Nusselt number (Nu) given by a numerical form using equation (3)

Nu=hL/k (3)

Further, thermal efficiency (ƞth) of solar air heater can be explained by using the equation

(4)

ƞth= qu/I=FR[I(τα)e-UL((Ti-To)/I] (4)

2.6.2. HYDRAULIC PERFORMANCE

It meets with the pressure drop (∆P) in the duct and accounts for energy consumption by a fan

to propel air through the duct. The non - dimensional form express it by using the equation (5)

Friction factor (f) = (∆P)Dh/2ρLV2

(5)

2.6.3. THERMO HYDRAULIC PERFORMANCE

The design of solar collector should be required for highest heat Energy transfer to the passing

fluid with the lowest consumption of fan energy, the design of solar collector should be

desirable and hence can be applies hydraulic and thermal characteristics of the collector

simultaneously. [4]

2.7. FACTORS AFFECTING THE FLOW PATTERNS ON RIB GEOMETRY

The characterization of roughness can be described by using the key dimensions geometrical

parameters are:

i. Rib Height (e): It generates the following effect on the roughness element

If e<δ There will be no roughness impact

If e>δ There will be more roughness impact of fluid pressure as compared to heat

transfer

If e>δ There will improve in heat transfer and moderate fluid pressure could be

served

ii. Rib Pitch (p): The flow on downstream side of the roughness element is divided

based on height and to maintain the pitch properly the reattachment of the flow

should be occurring. If the pitch ratio is less than 8 then the reattachment of the

shear layer does not occur and it which shows in poor heat transfer from the

surface.

iii. Rib Alignment and Inclination(α): The achievement of the soalr air heater in the

surface the friction factor goes down rapidly as the angle of attack fall down from

900 to 15

0.



iv. Duct Aspect Ratio (W/H): This factor plays a very important role in identifying

thermo hydraulic performance, which is the ratio of duct width to duct height.

v. Duct height (h): The air velocity can be increased by decreasing of duct height.

The higher efficiency of solar heater can be calculated by decreasing duct height.

By decreasing the depth solar air heater, efficiency can be increased. It is the ratio

of the distance between two consecutive ribs and height of the rib.

vi. Relative roughness height (e/Dh): It is defined as the ratio of rib height to

equivalent diameter of the air passage. The friction factor and Nusselt Number can

be increased by raising the relative roughness height.

vii. Angle of attack (α): It can be defined as the inclination of rib with direction of air

flow in the duct. The angling of the rib as per flow develop counter rotating

secondary flow along the span that reasons span wise variation of heat transfer

coefficient.

viii. The Shape of roughness element: The roughness elements can be written as two-

dimensional ribs or three-dimensional discrete elements, transverse or inclined ribs

or V-shaped continuous or broken ribs with or without a gap. It can be arc-shaped

wire, dimple, cavity, or compound rib-grooved. To investigate thermo hydraulic

performance the different shapes like circular, semi-circular and chamfered but the

most common shape of the ribs is square have been applied .

ix. Effect of rib chamfering: By deflecting the flow, the reattachment length can be

felt down and to reattach it closer to the rib. By decreasing the reattachment, the

length allows to organize ribs more nearly. Chamfering of the rib also enhances

the shedding of vortices developed at the rib top that show in raise turbulence.

Based on thermodynamic performance the optimum-chamfering angle has been

answered equal to 15-180.

3. TYPES OF ARTIFICIAL ROUGHNESS

3.1. Transverse continuous ribs

Prasad and Saini [5] experimentally researched, with small-diameter protrusion wire on

absorber plate, heat transfer coefficient and the friction factor of the fully developed turbulent

flow in a solar air heater duct. The type and position of the geometry are shown in Fig.7.

It recognized that the average Nusselt number and average friction factor in the roughened

duct were at 2.10, 2.24, 2.38 and 3.08, 3.67, 4.25 times that of the smooth duct for relative

roughness height (e/Dh) of 0.020, 0.027 and 0.033 respectively. The greatest improvement of

heat transfer coefficient and friction factor were at 2.38 and 4.25 times than that of smooth

duct respectively.


Roughness: A Review


3.2. Transverse broken ribs

Sahu and Bhagoria [14] studied the effect with 90⁰ broken transverse rib on the absorber plate

on the heat transfer coefficient and thermal Efficiency of solar air heater. Integral Rib

Roughened absorber plate was made by fixing wire of 1.5mm diameter over one side of the

absorber plate as depicted in Fig.8. With the roughness geometry having pitch (p) range from

10-30 mm, Reynolds number (Re) of 3000-12000, height of the rib (e) of 1.5mm, and a duct

aspect ratio (W/H) was 8. The greatest improvement of heat transfer coefficient occurs at a

pitch (p) of about 20mm while on another side of this pitch, the Nusselt Number decreases.

Varun et al. [22] research and reviewed the thermal aperture of solar air heater having a

roughness element as a combination of inclined as well as transverse rib with the relative

roughness pitch (p/e) of 3-8, Reynolds number (Re) ranges from 2000-14000, and relative

roughness height (e/Dh) of 0.030. It was analyzed that the most favorable thermal

performance occurs having the value of relative roughness pitch (p/e) of 8.

3.3. Inclined ribs

Kumar et al. [26] experimentally studied have been carried out with artificial roughness in the

form 600 discrete the inclined ribs for the improvement of heat transfer coefficient of a solar

air heater having roughened air duct provided. Marked improvement in heat transfer

coefficient has been reached with such roughness element. The investigated geometry has

been depicted in Fig.9.

Aharwal et al. [18] research and reviewed the effect of artificial roughness by inclined

split rib arrangement in a rectangular duct in solar air heater depicted in Fig.10. Considering

the gap of width ratio (g/e) and a gap of position ratio (d/w). The gain in Nusselt number and

the friction factor were in the range of 1.48-2.59 times and 2.26- 2.9 times of smooth duct

respectively.



Gupta et al. [6] research and reviewed on fluid flow and heat transfer characteristics of

artificially roughened solar air heater ducts with inclined wires. They found that the greatest

heat transfer coefficient appeared for an angle of attack (α) of 600 whereas the friction factor

greatest for an angle of attack of 700.

3.4. Wire Mesh

3.4.1. Expanded Wire Mesh Fixation

Saini and Saini [7] experiments carried out by providing the expanded metal mesh geometry

on the absorber plate in solar air heater, the effect of heat transfer coefficient and friction

factor as depicted in Fig.11. For fully matured turbulent flow in a rectangular duct with a

large aspect ratio (W/H) of 11.1 has been found greatest Nusselt number and friction factor

corresponding to the relatively long way length of mesh (l/e) and the relatively short way

length of mesh (s/e) was 46.87, 71.87 and 25, 15 respectively. The greatest improvement in

Nusselt number and friction factor values were reported of the order of 4 and 5 times to that

of the smooth absorber plate respectively.

3.4.2. Discredited metal wire mesh

Karmare and Tikekar [17] Examined experimentally with metal mesh grit roughness as shown

in Fig.12. The range of the parameter was the Reynolds number as 4000-17000, (e/Dh) as

0.035-0.044, (p/e) as 12.5-36 and (l/s) as 1.72-01. They displayed that plate with Roughness

parameter (l/s) as 1.72, (p/e) as 17.5 had optimum performance.


Roughness: A Review


3.5. Chamfered ribs

Karwa et al.[10] Examined experimentally for repeated integral chamfered rib roughness on

the absorber plate as depicted in Fig. 13. its effect with rib chamfered angle of 15⁰ to 18⁰

having an aspect ratio (W/H) of 4.8 to 12, Reynolds number (Re) of 3000 to 20000 relative

roughness pitch (p/e) of 4.5 to 8.5, and a relative roughness height (e/Dh) of 0.0141 to 0.0328.

The greatest improvement was found in the air heater with the highest relative roughness

height.

3.6. Wedge shaped ribs

Bhagoria [12] Examined Experimentally with wedge shaped transverse integral ribs, the

effect of relative roughness pitch (p/e) relative roughness height (e/Dh) and wedge angle (ɸ)

on the heat transfer coefficient and the friction factor in solar air heater rectangular duct

roughened depicted in Fig.14. The greatest improvement of heat transfer at wedge angle of

about 10⁰. The researcher recorded an enhanced in Nusselt number up to 2.4 times while the

friction factor increases up to 5.3 times as compared to a smooth duct.

3.7. W-shaped rib

3.7.1. Continuous W-ribs

Lanjewar et al.[27] Researched Experimentally with W-shaped rib by utilizing the concept of

increasing number of a secondary cell. The Range of the parameter was the relative roughness

height (e/Dh) as 0.018-0.03375, angle of attack (α) 30-750 and relative roughness pitch (p/e)

10. They noted a W-down arrangement with the angle of attack 600 given a most favorable

thermo-hydraulic performance. The greatest improvement of Nusselt number and friction

factor was 2.36 and 2.01 times that of the smooth plate for an angle of attack 600. Roughness

geometry is depicted in Fig.15.



3.7.2. Discrete W-ribs

Kumar et al. [20] investigated experimentally with discrete W-shaped ribs to find the heat

transfer distributions in solar air heater having its absorber plate roughened. The experiment

encircle Reynolds number (Re) range from 3000 to 15,000, rib height (e) values of 0.75 mm

and 1 mm, angle of attack (α) 450 relative roughness height (e/Dh) 0.0168 and 0.0225 and

relative roughness pitch (p/e) of 10.Thermal performance of the roughened solar air collector

was compared with that of smooth one under similar flow conditions and it was recorded that

the thermal performance of the roughened channel was1.2–1.8 times the smooth channel for a

range of parameters investigated & depicted in Fig.16.

4. ROUGHNESS ELEMENT COMBINATION

4.1. Transverse and inclined rib combination

Varun et al. [22] examined by applying the concept of combination roughness of transverse

and inclined ribs. They noted Reynolds number from 2000-14,000, relative roughness height

(e/Dh) as 0.030, relative roughness pitch (p/e) 3-8, they also reported that roughened collector

having roughness pitch (p) of 8 gave the best performance. Roughness geometry is depicted in

Fig.17.


Roughness: A Review


4.2. Transverse rib groove combination

Jaurker et al. [15] researched experimenting with performance of transverse rib roughness for

enhanced efficiency. The greatest heat transfer was achieved for relative roughness pitch (p/e)

of 6. The most favorable heat transfer was recorded in groove position to pitch ratio of 0.4.

Roughness geometry is depicted in Fig.18.

4.3. Chamfered rib groove combination

Layek et al. [16] examined with chamfered rib roughness. Experimental study was carried out

for Reynolds number from 2000-21,000, relative roughness pitch (p/e) 4.5-10, chamfer angle

(α) as 5-300, relative groove position (g/p) as 0.3-0.6, and relative roughness height as (e/Dh)

0.019-0.043.They recorded the Nusselt number and friction factor increased by 3.24-0.78

times, respectively, as compared to a smooth duct. The greatest improvement of Nusselt

number and friction factor were achieved for the relative groove position (g/p) of 0.4.


4.4. Arc shaped ribs

Saini and Saini [19] researched and Reviewed with providing the bow- shaped parallel wire

on the absorber plate in solar air heater,the effect of heat transfer coefficient and friction

factor of relative roughness height (e/Dh) and angle of attack (α/90) as depicted in Fig. 20.

With Reynolds, number (Re) ranges of 2000-17000, angle of attack (α/90) of 0.3333-0.6666

and relative roughness height (e/Dh) of 0.0213-0.0422 for a fixed relative roughness pitch

(p/e) of 10. The greatest improvement in Nusselt number was achieved as 3.80 times

corresponding to the relative angle of attack (α/90) of 0.3333 at the relative roughness height

(e/Dh) of 0.0422.



4.5. Dimple surface

4.5.1. Transverse dimple roughness

Saini et. al [21] used the latest concept of dimple shaped artificial roughness. Experimentally

enclosed range of parameters are: Reynolds number (Re) from 2000-12,000, relative

roughness height (e/Dh) as 0.018-0.037 and relative roughness pitch (p/e) 8-12. They found

the greatest value of relative roughness height of 0.0379 and relative roughness pitch of 10.

The least value of friction factor corresponding to the relative roughness height as 0.0289,

relative roughness pitch of 10. Roughness geometry is depicted in Fig. 21

4.5.2. Staggered dimple roughness

Bhushan et al. [28] studied with staggered dimple roughness in place of transverse dimple

roughness. Range of parameter reviewed were relative short way length (s/e) as 18.75-37.50,

relative long way length (l/e) as 25.00-37.50, Reynolds number (Re) from 4000-20,000,

relative print diameter (d/D) as 0.147-0.367, relative roughness height (e/Dh) as 0.03, an an

aspect ratio (W/H) as 10. Greater improvement in heat transfer coefficient was recorded for

relative short way length (s/e) of 31.25, relative long wavelength (l/e) of 31.25 and relative

print diameter (d/D) of 0.294 & depicted in Fig. 22.

4.5.3. Arc shaped dimple roughness 1

Yadav et al. [42] reviewed with arc shaped dimple roughness. Investigated parameter was

Reynolds number range from 3600-18,000, arc angle of protrusion arrangement as 45-750

(p/e) as 12 to 24, and (e/Dh) as 0.015-0.03 .The greatest improvement of Nusselt number and

friction factor was found to be 2.89 and 2.93 times, respectively of smooth duct for a range of


Roughness: A Review


parameter examined. The greatest improvement of the high heat transfer and friction factor

occurred in relative roughness height (e/Dh) of 0.03, relative roughness pitch (p/e) of 12 and

for arc angle value of 600. Roughness geometry is shown in Fig. 23.

4.5.4. Arc shaped dimple roughness 2

Sethi et al. [30] examined dimple shaped roughness. The range of parameters was investigated

and covered by duct aspect ratio (W/H) 11, relative angle of attack 47-750, Reynolds number

(Re) from 3600-18,000, relative roughness height (e/Dh) as 0.021-0.036,. They recorded

greatest value of the Nusselt number corresponding to the relative roughness height (e/Dh) of

0.036, relative roughness pitch (p/e) of 10 and arc angle 600 as depicted in Fig. 24.

Singh et al. [33] experimentally reviewed enclose the Reynolds number (Re) in the range

of 2200–22,000, relative roughness width (W/w) ranges from 1 to 7, relative roughness pitch

(p/e) the range of 4–16 relative roughness height (e/Dh) values of 0.018–0.045, and arc angle

(α) range of 30–750. By investigation and accumulated data correlations for the Nusselt

number and friction factor were matured as shown in Fig.25.

Pandey et al. [35] conducted analytically with multiple arcs with gaps.The range of

parameters were recorded are Reynolds number (Re) 2100-21000, relative roughness height



(e/Dh) =0.016 to 0.044, angle of attack (α) =30-750 Relative width (W/w) =1-7, relative gap

distance (d/α) =0.25-0.85, relative roughness pitch (p/e) =4-16,.The greatest increase in new

and off was5. 85 and 4.96 times in comparison to the smooth duct are shown in Fig. 26.

4.6. V-Shaped ribs

4.6.1. Continuous type

Momin et al. [11] examined with inclined rib resulted in improve performance than transverse

ribs due to the increase of secondary vortices. The number of secondary vortices was

increased. They reviewed with V-shape rib roughness as depicted in Fig. 27. And also

examined the thermo hydraulic performance of solar air heater for Reynolds number (Re) as

2500-18,000, relative roughness height (e/Dh) as 0.02-0.034, an angle of attack (α) as 30-90

for fixed relative roughness pitch (p/e) of 10. Nussselt number and friction factor were

recorded as 2.30 and 2.83 times of smooth duct plate for the angle of attack 600 for greatest

improvement.

4.6.2. Discrete Type

Muluwork et al. [8] investigated and compared the thermal performance of V-shaped rib with

the staggered discrete V-apex up and V-down the ribs with corresponding staggered discrete

ribs. The roughness geometry is depicted in fig.28. They determined a Stanton number for V-

down discrete ribs greater than corresponding V-up and transverse discrete ribs. Stanton

number recorded improvement as 1.32-2.47 in the range of parameters examined.


Roughness: A Review


Sahu et al. [34] examined experimentally with Discrete arc-shaped rib roughness, heat

transfer and friction factor in solar Air heater duct on absorber plate, the roughened wall being

heated while the remaining three walls are insulated. It observed that the Nusselt number

increases as increases the Reynolds number attains a maximum for pitch (p) of 15 mm. For

improving the efficiency of solar air heater height of the rib in the range of 1-2mm, duct

aspect ratio (W/H) =8, relative roughness pitch (p/e) =10, an angle of arc 300, relative angle of

attack (α/90)0.3333, was utilized as depicted in Fig. 29.

Karwa et al. [13] experimental reviewed by V-discontinuous and V- discrete ribs. The

range of the parameter was relative roughness pitch (p/e) as 10.62, relative roughness length

(B/S) as 3 and 6, angle of attack as 450 and 600 and a Reynolds number as 2850-15,500.

They determined that discrete ribs perform greater than discontinuous ribs and 600 rib

perform better than 450 ribs. Roughness geometry is depicted in Fig. 30 & 31.



4.7. Multiple V-ribs

4.7.1. Multiple continuous V-ribs

Hans et al. [25] examined with multiple continuous V- ribs and using a concept of increasing

number of secondary flow cells. The experiment encircled with Reynolds number (Re) from

2000-20,000, angle of attack 30-750, relative roughness height as (e/Dh) 0.019-0.043, relative

roughness pitch (p/e) 6-12, and relative roughness width (W/w) range as 1-10. Greatest heat

transfer occurred for relative roughness width (W/w) of 6 while friction factor attained

greatest value for relative roughness width (W/w) of 10. Both Nusselt number and friction

factor 6 and 5 times respectively in comparison to a smooth duct of parameter investigated.


4.7.2. Multiple V-ribs with gap

Kumar et al. [31] utilized and examined the method of turbulence and acceleration of flow by

providing gap. Range of parameter encircled are Reynolds number (Re) from 2000-20,000,

relative width ratio as 6, angle of attack (α) 600, the relative gap distance ratio (Gd/Lv) as

0.24-0.8, relative gap width (g/e) as 0.5-1.5, relative roughness height (e/Dh) as 0.043. They

recorded the greatest improvement in Nusselt number and friction factor as 6.32 and 6.12

times of a smooth plate respectively as depicted in Fig. 33.

Maithani et al. [38] Researched experimentally have been carried out for improvement of

heat transfer coefficient with V-ribs and symmetrical gaps as turbulence promoter was used.

The investigated encircled Reynolds number (Re) 4000-18000, angle of attack (α) =30-750,

gap width to rib height (g/e) =1-5, relative roughness height (e/Dh) =0.043, relative

roughness pitch (p/e) =6-12, Number of gaps (Ng) =1-5.The greatest improvement of the

order of 3.6 times that of the smooth duct has been achieved, similarly, friction factor(ff ) also

increase by 3.67 times of that of smooth duct achieved as described in Fig. 34.


Roughness: A Review


Kumar et al. [39] Experimentally and numerically analyzed of heat transfer improvement

with broken multiple V-type baffle encircled parameters of channel aspect ratio (WD/HD)

=10, Reyonlds No. (Re) =3000-8000, relative discrete distance (DD) 0.67, relative baffle

width (WD/WB) 1-6, relative baffle height (HB/HD) = 0.5, relative baffle pitch (PB/HB) =

10, relative gap width (g/e) =1 with 600 angled broken multiple V-type baffles. The obtained

experimental results showed that greatest overall thermal performance occurred at a relatively

baffle width of 5, as depicted in Fig. 35

Kumar et al.[40] Researched and Examined with ‘S’ shape ribs for heat transfer and

friction factor and their correlations development for solar air heater duct artificially

roughened with aspect ratio(W/H)=12, , arc angle (α) =30-750,relative roughness width

(W/w) =1-4, relative roughness pitch(p/e)=4-16,relative roughness height(e/Dh)=0.022-0.054

and Reynolds number (Re) =2400-20000. Experimental results showed that the greatest

improvement in Nusselt number (Nu) and friction factor (f) have been found in relative

roughness width (W/w) =3, relative roughness pitch (p/e) =8, relative roughness height (e/Dh)

=0.043, arc angle (α) =600 as depicted in Fig.36



6. CONCLUSION

In this paper, an endeavor has been made to report the heat transfer coefficient and friction

factor characteristics of artificially roughened duct in solar air heater utilizing of an alternate

type of rib geometry. Tentatively Investigations completed for utilization of manufactured of

artificial roughness of different shapes, size and orientation by various researchers in order to

improve the heat transfer.

In light of the writing audit the accompanying Conclusion is drawn:-

1. Artificial roughness utilized as a part of solar air heater are a powerful method to

improve heat transfer to the liquid streaming having in the pipe and it announced

the greatest heat transfer contrasted with the smooth surface under a similar

parameter/working conditions. Distinctive sorts of roughness geometry utilized as

a part of solar air heater are inferring that it relies on shapes, size, orientation and

arrangement of the roughness component on the absorber plate.

2. There is a positive increment in heat transfer in solar air heater with an expansion

in erosion to the stream when its surface is roughened. In any case, the diverse

specialists locate an alternate estimation of the addition in heat transfer and

rubbing factor for each kind of rib geometry utilized.

3. Many sorts of parameter that recognized the roughness elements, but most

repeated roughness geometry for solar air heater is repeated ribbing type which is

described by the dimensionless parameter .i e. Relative roughness pitch (P/e),

relative roughness height (e/Dh) angle of attack (α), channel aspect ratio (W/H).

High perspective, proportion esteems have better heat transfer effectiveness while

low viewpoint proportion esteems have a superior heat transfer execution in solar

air heater and pipes.

4. In transverse rib component, the vast majority of the examinations directed give

the outcome that the execution of the V-shaped, W-shaped ribs is ideal at how the

attack angle of 600, though for transverse inclined ribs the ideal execution edge of

slant is 450. Use of multi V-formed rib roughness with gap has the most elevated

Nu when contrasted with different roughness geometries for the explored scope of

parameters.

5. It was discovered that utilization of the arc shaped formed rib geometry and metal

grit ribs have the most astounding thermo-hydraulic performance parameters when

contrasted with regularly roughness geometry for the investigated scope of

parameters. The utilization of broken arc ribs has the greatest Nu contrasted with

the basic arc based molded rib roughness for the examined range of parameters.

6. The utilization of V-sort showed demonstrated that highest overall thermal

performance execution happened in a moderately perplex width contrasted with

circular segment molded with various design and V type discrete formed.

7. In the outline of solar air heater, Computational fluid dynamics approach has been

applied as a decent approach as of late and it is to improve the plan procedure

arrangements with the heat transfer and fluid flow.

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