Pressure drop and heat transfer of flow boiling R134a in a dimpled flat duct

A test facility has been built up to investigate the pressure drop and heat transfer of flow boiling R134a in a dimpled flat duct. The experiments are conducted at mass flux from 102.2 to 205.0 kg m⁻² s⁻¹, heat flux from 2.3 to 9.3 kW m⁻², pressure from 349.8 to 488.4 kPa and vapor quality from 0.05 to 0.95. The results show that the frictional pressure gradient increases with increasing vapor quality and mass flux, while decreases with a rise in pressure. The heat flux has little impact on the frictional pressure gradient. The flow boiling heat transfer coefficient decreases slightly first and then increases significantly with an increase in vapor quality, which is attributed to the combined contributions of nucleate boiling heat transfer and convective evaporation heat transfer. No dryout has been observed in the tested vapor quality region. The flow boiling heat transfer coefficient visibly increases with increasing mass flux and heat flux, while the rise in pressure just slightly increases the flow boiling heat transfer coefficient in low vapor quality region only. The comparisons with correlations for regular channels show that the dimpled flat duct has 3.9 to 8.3 times frictional pressure gradient and 1.3 to 3.0 times heat transfer coefficient.