ChemEngDayUK 2015

This study is focused on a 2D test element (water pipe) wherein a

compressed water flow (300 bar) is heated up by a constant

temperature applied on the pipe walls, in order to make the fluid

reach the supercritical state and surpass the pseudocritical line (at

401.9° C approx.).

Introduction

Both heat transfer conduction (pipe) and convection (flow) are

considered. The SST (Shear Stress Transport) turbulence model is

being utilised with no-slip conditions (low Reynolds, takes the

interphase wall-flow into consideration instead of wall functions).

A mapped mesh was chosen for the flow, with refined interface:

Computational methods

Conjugate Heat Transfer Study of Supercritical Water

in a Boiler Pipe by Modelling with Comsol Multiphysics A. Gil-Garcia*, I. Kings, B. Al-Duri

University of Birmingham, School of Chemical Engineering, Edgbaston, Birmingham B15 2TT, UK *email: AAG278@bham.ack.uk

Figure 1. Computational domain for the 2D test element

Figure 2. Boundary conditions for the 2D test element

Figure 3. Upper half of the test element showing the wall-fluid interface (axes numbers in mm.)

Results

• Similar HTC behaviour was obtained on the first half of the pipe,

compared to other models in literature with the same conditions [2].

• Downward flow shows the best thermal behaviour in terms of HTC.

• The upward flow shows several stages of deterioration and

restoration, and seems more sensitive to angle variations mostly

from 1 to 15° due to buoyancy increasing its effect on the first half.

• Overall, strong deterioration was found due to a high q/G ratio.

Conclusions

[1] Sarah Mokry et al. Development of supercritical water heat-transfer correlation for vertical

bare tubes. Nuclear Engineering and Design 241 (2011) 1126–1136.

[2] J.A.M. Withag et al. Heat transfer characteristics of supercritical water in a tube: Application

for 2D and an experimental validation. J. of Supercritical Fluids 70 (2012) 156– 170.

[3] Jackson, J.D., 2002. Consideration of the heat transfer properties of supercritical pressure

water in connection with the cooling of advanced nuclear reactors. Proceedings of the 13th

Pacific Basin Nuclear Conference, Shenzhen City, China, October 21–25.

References

• Pipe inner diameter: 10 mm • Outer wall temperature: 600° C

• Pipe thickness: 3 mm • Water inlet temperature: 350° C

• Pipe length: 2500 mm • Water mass flux: 200 kg/m2s

• Heated length: 2100 mm • Upwards, downwards, horizontal

Supercritical coal-fired power plants operate at thermal efficiencies up to 10% higher than conventional (subcritical) plants thanks to

their improved thermodynamic behaviour. However, their dynamic response in case of a fast increase in energy demand must be

evaluated for compliance with the Grid Code.

Using the “Conjugate Heat Transfer” module in Comsol Multiphysics, validation with the literature was firstly carried and the

convective heat transfer coefficient (HTC) was modelled afterwards with correlations [1] in order to research its improvement (IHT) or

deterioration (HTD) depending on the conditions (heat flux, mass flux and pipe orientation).

Abstract

Figure 4. Left: HTC value from [2] (90° upwards). Right: HTC values at various upward angles.

Figure 6. Ratio of supercritical HTC over several standard (subcritical) HTC’s to show HTD & IHT zones.

Figure 5. HTC values at various downward angles.