Mathematical and CFD modeling for a rectangular finned tube adsorption bed for automotive cooling system

Transcription

1 SusTEM Special Sessions on Thermal Energy Management Mathematical and CFD modeling for a rectangular finned tube adsorption bed for automotive cooling system B. Shi, R.K. AL-Dadah, S. Mahmoud, A. Elsayed and A. Rezk

2 Presentation outline Background Investigation aim Bed design Mathematical model CFD model CFD simulation results Result comparison Simulation results for desorption Effectoffinheightandfinpitch Conclusions

3 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

4 Background Operation concept of adsorption Chiller

5 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 adsorption and desorption processes Computational fluid technique to simulate the adsorption and desorption processes in the bed

6 Bed Configuration Mathematical model CFD model MATLAB with REFPROP COMSOL Multiphysics Bed design parameters Symbol Value Description L f 275mm Bed Length W f 115mm Fin Width H f 30mm Fin Height p f 1.5mm Fin Pitch t f 0.105mm Fin Thickness D o mm Tube Outer Diameter t t 0.8mm Tube Thickness Adsorption Conditions Condition Value Initial Bed Temperature 60ºC Evaporator Temperature 15ºC Cooling Water Inlet Temperature 30 ºC Cooling Water Flow Rate 30L/min Initial Bed Pressure kPa Initial Uptake Rate 0.02kgwater/kgsilicagel

7 Mathematical model Energy equation M s H sdw / dt + m Cpc ( Tin Tout ) + M s[ hw ( Te) hw ( Pe, Tbed )] dw / dt Adsorption kinetics : where and

8 Mathematical model Calculation for thermal resistance

9 CFD model

10 CFD model Darcy equation Energy equation Adsorption kinetics : where and

11 CFD simulation results Temperature distributions at 600s Temperature distribution of the last fin at 100s, 300s and 600s

12 Results comparison Deviation between two models Maximum deviation Water uptake rate 8.2% Average bed temperature 0.9% Water outlet temperature 0.2% Both techniques can predict the dynamics of the adsorption process with reasonable deviation

13 Simulation results for desorption Desorption Conditions Condition Value Initial Bed Temperature 30ºC Heating Water Inlet Temperature 60 ºC Heating Water Flow Rate 30L/min Initial Bed Pressure kPa Initial Uptake Rate 0.02kgwater/kgsilicagel Results from both methods indicate good agreement

14 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.

15 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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