Inlet flow effects in micro-channels in the laminar and transitional regimes on single-phase heat transfer coefficients and friction factors

An experimental investigation of heat transfer and pressure drop in rectangular micro-channels was conducted for water in the laminar and transitional regimes for three different inlet configurations. The inlet types under consideration were the sudden contraction, bellmouth, and swirl inlet types, and hydraulic diameters of 0.57, 0.85, and 1.05 mm were covered. It was found that the critical Reynolds number and the transitional behaviour in terms of heat transfer coefficients and friction factors were influenced significantly by the inlet type. For the sudden contraction inlet type, which were investigated for both adiabatic, as well as diabatic cases, adiabatic friction factors were predicted well by the laminar Shah and London correlation, while diabatic friction factors were decreased with an increase in wall heat flux. The sudden contraction inlet critical Reynolds numbers were found to be between 1800 and 2000 for adiabatic cases, while for diabatic cases the transition regime commenced at a Reynolds number of about 2000. The bellmouth and swirl inlet types were mostly investigated for diabatic cases only with swirl inlet tests limited to the 1.05 mm channel. Laminar friction factors were approximately similar to those of the sudden contraction inlet type, however, after the commencement of transition both inlet types exhibited higher friction factors than the sudden contraction inlet. Minor transition occurred as early as at Reynolds numbers of 1200 and 800 for the bellmouth and swirl inlet types respectively while major transition occurred at Reynolds numbers of approximately 1800 and 1500 respectively. Critical Reynolds numbers were found not be significantly influenced by the channel diameter to length ratio considered in this study. Laminar Nusselt numbers were predicted well by conventional macro-channel thermal entry correlations. The swirl inlet type exhibited the highest friction factors and Nusselt numbers in the transitional regime followed by the bellmouth inlet type. During transition while compared with the sudden contraction inlet, both the bellmouth and swirl inlet types exhibited larger enhancement in heat transfer than increases in the friction factor penalty. Based on the experimental data obtained in this study, a set of correlations were developed which describes the relation between the friction factor and Colburn j-factor. Depending on the inlet type, the correlations predicted between 94% and 100% of the results to within 10% of the experimental measurements.