Numerical Analysis of heat and mass transfer in Heat and Moisture Exchanger (HME)

Pezhman Payami1

Supervisors: Masud Behnia1, Barry Dixon2

1 Fluid Dynamics Group, School of Mechanical Engineering, 2 Saint Vincent’s Hospital, Melbourne

2

Contents

› Significance

› General Classification of HMEs

› Problem Specification

› Heat Transfer Mechanisms

› Methodology

› References

3

Significance

Normal breathing and nose function

To warm and humidify inspired air in

upper airways to reach the alveoli as

saturated vapour at the core temperature

To maintain core body temperature within

an appropriate range

To prevent drying of the tracheal mucosa

and other structures causing respiratory

mucosal dysfunction and hypothermia Upper airways and nose structure

4

Significance

Mechanically ventilated patients

When the upper airways are bypassed by oral or nasal endotracheal intubation it is

essential to seek an alternative way to heat and humidify inspiratory gases

HME is an artificial nose (passive humidifier) that traps expiratory heat and moisture in

a medium and returns a portion of it to the next inspiration

HME as an artificial nose in mechanically ventilated patients

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General Classification of HMEs

HMEs

Hygroscopic

Hydrophobic

Composite (Hygroscopic/Hydrophobic)

Composed of plastic foam, wool or paper condensation surfaces with a low thermal conductivity Impregnated with a hygroscopic chemical such as Calcium Chloride to improve moisture conserving properties

Large pleated surface composed of ceramic fibres Covered by a synthetic resin that repels the water

Felt filter layer such as polypropylene non-woven fibre subjected to an electrical field to improve filtration efficiency Moisture exchange component of polyurethane open-cell foam or cellulose fibre (either cotton or wood pulp) impregnated with Calcium Chloride

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Problem Specification

Patient side

(T=34ºC, RH=100%)

Ventilator side

Peak airway pressure:

less than 30 cmH2O

Flow rate:

30 l/min

Frequency:

12-16 times per minute

Temp and RH:

room air conditions could be assumed for the first run

› The flow is considered incompressible/ steady/ laminar

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Heat Transfer Mechanisms

Conduction

•Inside/between porous material and the casing

Convecti

on

•Between gas phase and solid portion of porous material

•Between the casing and the ambient air

Radiatio

n

•Radiation heat transfer across the domain (could be neglected)

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Methodology

Porous

Flow

Modified N-S equations using Darcy’s law

Linear directional loss defined by streamwise and transverse permeabilities

Heat transfer

Energy equation for solid phase

Interfacial heat transfer between the fluid and solid considering overall heat transfer coefficient between the fluid and the solid

Mass transfer Mass concentration of each component according to ideal gas equation of state

Fluid

Flow N-S equations

Heat transfer

Energy equation for fluid phase by considering porosity effect in the porous zone

Interfacial heat transfer between the fluid and solid considering overall heat transfer coefficient between the fluid and the solid

Mass transfer Mass concentration of each component according to ideal gas equation of state

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Methodology

General transport equation

Where is a general variable that can be replaced with macroscopic properties of the fluid such

as pressure, velocity components or temperature to describe the behavior of the flow

In the porous zone the Darcy’s law is governed by

A computational fluid dynamics package, ANSYS CFX 13, is used to simulate fluid flow and heat

transfer in the HME

Sgraddivdiv

t

u

Rate of increaseof of fluid

element

Net rate of flowof out of

fluid element(convection)

Rate of increaseof due to

diffusion

Rate of increase of due tosources

=+ +

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References

› Tariku F, Kumaran M.K., Fazio P., Transient Model for Coupled Heat, Air and Moisture Transfer Through Multilayered Porous Media, International Journal of Heat and Mass Transfer, 53, pp. 3035-3044, 2010.

› Baggio P., Bonacina C., Schrefler B.A., Some Considerations on Modelling Heat and Mass Transfer in Porous Media, Transport in Porous Media, 28, pp. 233-251, 1997.

› Kaya Ahmet, Aydin Orhan, Dincer Ibrahim, Numerical Modelling of Heat and Mass Transfer During Forced Convection Drying of Rectangular Moist Objects, International Journal of Heat and Mass Transfer, 49, pp. 3094-3103, 2006.

› R. Younsi R., Kocaefe D., Poncsak S., Kocaefe Y., Gastonguay L., CFD Modelling and Experimental Validation of Heat and Mass Transfer in Wood Poles Subjected to High Temperatures: a Conjugate Approach, International Journal of Heat and Mass Transfer, 44, pp. 1497-1509, 2008.

› Eva Barreira, João Delgado, Nuno Ramos and Vasco Freitas (2010). Hygrothermal Numerical Simulation: Application in Moisture Damage Prevention, Numerical Simulations - Examples and Applications in Computational Fluid Dynamics, Lutz Angermann (Ed.), ISBN: 978-953-307-153-4, InTech, Available from: http://www.intechopen.com/articles/show/title/hygrothermal-numerical-simulation-application-in-moisture-damage-prevention

› Dellamonica J., Boisseau N., Goubaux B., Raucoules-Aime M., Comparison of Manufacturers’ Specifications for 44 Types of Heat and Moisture Exchanging Filters, British Journal of Anaesthesia, 93 (4), pp. 532-539, 2004.

› ANSYS, ANSYS CFX-Solver Theory Guide. 2010, Canonsburg, PA