Table 1 Uncertainties for different parameters involved in the experimental tests Parameter Uncertainty Temperature, T (°C) ±0.1°C Mass flow rate, (kg/s) ±1.3% Mass flux, G (kg/m2s) ±1.35% Position of thermocouples, y (m) ±0.1 mm Power
input, (W) 1% Heat flux, q (W/m2) 8% Heat transfer coefficient, h (W/m2k) ±12% Results and discussion Experiments are performed in parallel rectangular minichannels using pure water and Combretastatin A4 silver-water nanofluid with two small volume fractions (0.000237% and 0.000475%) as working fluids in a compact heat exchanger. A comparison between proposed correlations in the literature and experimental www.selleckchem.com/products/Vorinostat-saha.html data is carried out initially to verify the present measurements and then to evaluate correlations defined for flow boiling heat transfer in minichannel or macrochannel. Experiments are conducted with various values of mass flux and heat flux. Water boiling heat transfer in minichannels: measurement results and predictions Transient state: temperature measurements and instability For each operating conditions, wall
temperatures are measured at different axial locations of the minichannels. Figure 5a shows an example of four transient temperatures profiles measured at 0.5 mm below the heat exchange surface along the flow direction. The experiment is conducted for 60°C inlet water temperature, 266 kg/m2s mass flux GSI-IX solubility dmso and 200 W supplied power to the heated plate. The figure shows that the wall temperatures increase regularly during transient state with some fluctuations (Figure 5b) until a limit is reached then decrease at the start of the nucleate boiling to reach steady values. Figure 5b shows an example of the wall temperature fluctuations in the steady state zone caused by the hydrodynamic instabilities of the bubbles and liquid flows. In a previous work, it was revealed that various types of hydrodynamic instabilities may exist in boiling flow and boiling flow has a destabilizing effect on the two-phase flow. In this study, experimental data show that bubbles generated on the heated surface move to the channel
exit and coalesce with other bubbles to feed the high void fraction. Flow oscillation in the minichannels may be attributed to the difference between the vapor and the liquid densities. PAK5 Instability in boiling flow can reduce the critical heat flux due to the flow oscillation that tends to increase the bubble velocity along the channel. Previously, Qu and Mudawar [4] showed that pressure drop oscillation is undesirable for the performance of a two-phase microchannel heat sink. Figure 5 Evolution of the wall temperature. (a) Measurements by various thermocouples along the flow direction for 0.5 mm depth and (b) example of wall temperature fluctuations. Steady state: temperature and heat transfer coefficient measurements Figure 6a,b shows an example of the wall temperature measured at 0.5 and 8 mm below the heat exchange surface for the channels 1 and 41 for 348 kg/m2s pure water mass flux.