B. Shi, R.K. AL-Dadah, S. Mahmoud, A. Elsayed and A. Rezk

Mathematical and CFD modeling for a

rectangular finned tube adsorption bed for

automotive cooling system

SusTEM Special Sessions on

Thermal Energy Management

Presentation outline

• Background

• Investigation aim

• Bed design

• Mathematical model

• CFD model

• CFD simulation results

• Result comparison

• Simulation results for desorption

• Effect of fin height and fin pitch

• Conclusions

Background

• Adsorption cooling systems in automotive applications

– Driven by engine coolant water or engine exhaust gas

• Decreasing fuel consumption

• Decreasing exhaust gas emissions

– Limited application because of large volume, high weight and low efficiency

• Advantages of water refrigerant

– High latent heat of evaporation

– Thermal stability at high temperature

– Good compatibility with wide range of materials

• Advantages of silica gel adsorbent

– Excellent thermal properties

– Low generation temperature

– High adsorption kinetics

Background

Operation concept of adsorption Chiller

Investigation aim

• This paper developed two simulation methods to investigate the

adsorption and desorption processes of a silica gel-water pair in

rectangular finned tube bed structure that can be used for automotive

application

– The lumped parameter approach to model the heat and mass transfer in the adsorptionand desorption processes

– Computational fluid technique to simulate the adsorption and desorption processes inthe bed

Symbol Value Description

Lf 275mm Bed Length

Wf 115mm Fin Width

Hf 30mm Fin Height

pf 1.5mm Fin Pitch

tf 0.105mm Fin Thickness

Do 15.875mm Tube Outer Diameter

tt 0.8mm Tube Thickness

Bed design parametersCondition Value

Initial Bed Temperature 60ºC

Evaporator Temperature 15ºC

Cooling Water Inlet Temperature 30 ºC

Cooling Water Flow Rate 30L/min

Initial Bed Pressure 1.01325kPa

Initial Uptake Rate 0.02kgwater/kgsilicagel

Adsorption Conditions

• CFD model

MATLAB with REFPROP

COMSOL Multiphysics

Bed Configuration

• Mathematical model

Mathematical model

where and

• Energy equation

• Adsorption kinetics :

dtdwTPhTehMTTCpmdtdwHM bedewwsoutincss /)],()([)(/ −+−+∆•

Mathematical model

• Calculation for thermal resistance

CFD model

CFD model

where and

• Adsorption kinetics :

• Energy equation

• Darcy equation

CFD simulation results

Temperature distributions at 600s

Temperature distribution of the last fin at 100s, 300s and 600s

Results comparison

Maximum deviation

Water uptake rate 8.2%

Average bed temperature 0.9%

Water outlet temperature 0.2%

Deviation between two models

• Both techniques can predict the dynamics of the adsorption process with reasonable deviation

Simulation results for desorption

• Results from both methods indicate good agreement

Condition Value

Initial Bed Temperature 30ºC

Heating Water Inlet Temperature 60 ºC

Heating Water Flow Rate 30L/min

Initial Bed Pressure 1.01325kPa

Initial Uptake Rate 0.02kgwater/kgsilicagel

Desorption Conditions

Effect of fin height and fin pitch-CFD Investigation

Different fin height Different fin pitch

• Decreasing the fin height results in increasing the water uptake rate by 19% from 35mm to 20mm.

• Decreasing the fin pitch results in increasing the water uptake rate by up to 8% from 3mm to 1.5mm.

Conclusions

• A mathematical model and a CFD model were developed to investigate the performance of a silica gel-water pair in rectangular finned tube bed structure that can be used for automotive application

• Simulations were used to investigate the bed performance of adsorption and desorption performance with results form both methods were very close.

• The CFD technique was used to investigate the effect of fin pitch and fin height on the performance of adsorption bed

– Decreasing the fin height results in increasing the water uptake rate

– Decreasing the fin pitch results in increasing the water uptake rate

• 12 beds (distributed between two adsorbers) will be required to provide an average of 2kW for an automotive air conditioning system

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