Numerical Analysis of Parametric Effects of Tube-Strip Heat Exchanger for Fuel Cell Vehicles

Fuel cell vehicles (FCVs) are facing more severe heat dissipation challenges since the fuel cell stack is required to operate at a lower temperature and thus smaller heat exchanger temperature difference. Thorough analysis of the parametric effects is required to maximize the thermal performance. In this paper, a numerical analysis of the tube-strip heat exchanger is conducted for the targeted application in a high performance passenger FCV. The representative unit cell is used to model the detailed fluid flow and heat transfer at both hot and cold sides in the theoretical framework of volume averaging. Based on the numerical computation over the representative unit cells, the flow, temperature and pressure fields are obtained, which are then utilized to obtain the cell-level heat transfer coefficient between the hot and cold fluids. The obtained heat transfer coefficients are used for the estimation of the heat exchanger thermal performance based on the effectiveness-NTU method. Different air and liquid water flow rates are first examined. Various design parameters such as fin height, fin spacing, fin thickness and fin material are examined through the heat transfer analysis at the unit cell level. Attention is also paid on the improvement of the air-side performance by changing fin shapes to increase the heat transfer coefficient of the heat exchanger. The result shows that the total exchanged heat of the aluminum louvered fin heat exchanger, with fin thickness of 0.06mm, fin height of 5mm and fin spacing of 1mm, can reach 59.24 kW at the liquid flowrate of 120L/min and air velocity of 5m/s.

Air-Side Thermal and Drain Performance of Wire-Strip Fin-Tube Heat Exchanger

The purpose of the present study is to investigate the characteristics of pressure drop, heat transfer rate and drainability of a wire-strip heat exchanger modified from the conventional louver fin one. Scaled-up models with the standard and four kinds of wire-strip fins were made, and their thermal and hydraulic performances tested in a wind tunnel. The drainability of the fins was evaluated by measuring the water retained after dipping the heat exchangers. The f and j factors and their ratios for the wire-strip fins are compared with those of the conventional louver fin, and empirical correlations and theoretical predictions were made. The best design for the wire-strip fins showed a better thermal performance per unit fan power than the standard louver fin by several tens of percent. It also retained about half or less of the water.