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This study presents the single-phase heat transfer and pressure drop characteristics of R1233zd(e) in a Brazed Plate Heat Exchanger (BPHE). Experiments on single-phase, water-to-water, were initially conducted and a correlation for the convective heat transfer coefficient of the hot water side was derived by applying the modified Wilson plot method. The experiments covered a range of Reynolds number from 80 to 1600 and Prandtl number from 2.8 to 7.0. Subsequent experiments were conducted with water-to-R1233zd(e) covering a refrigerant range of Reynolds number from 700 to 1450 and Prandtl number from 4.5 to 4.9. The results were used to assess existing correlations in the literature predicting the Nusselt number and Fanning friction factor in BPHEs. Finally, new correlations for both the hot (water) and cold (refrigerant) sides are proposed for single-phase heat transfer for this geometry covering the conditions above. The proposed refrigerant heat transfer correlation predicted 97% of all data within the ± 10% error bands at a mean absolute error value of 5.7%. [ABSTRACT FROM AUTHOR]

• Flow boiling study of HFE-7100 in multi-microchannels. • Flow patterns, pressure drop and heat transfer rates in microchannel evaporator. • The heat transfer coefficient increased with heat flux and did not depend on mass flux. • The present data were compared with existing correlations. The main objective of this research was the design and test of a multi-microchannels heat sink for electronics cooling applications, which operates at system pressure near atmospheric and low mass flow rates. HFE-7100, a dielectric and eco-friendly coolant, was chosen as the working fluid. Twenty five rectangular microchannels, of width 0.7 mm and height 0.35 mm giving a hydraulic diameter of 0.46 mm, were fabricated from oxygen-free copper with a base area of 500 mm2. The channels in-between wall was 0.1 mm thick. Five mass fluxes ranging from 50 to 250 kg/m2 s were tested at fixed inlet sub-cooling near 5 K. The effect of heat flux, mass flux and vapour quality on the local heat transfer coefficient was investigated. Four flow patterns namely; bubbly, slug, churn and annular flow, were visualized using a high-speed camera mounted on a microscope. In this study, the maximum flow boiling heat transfer coefficient was 12.71 kW/m2 K at mass flux of 250 kg/m2 s. A comprehensive comparison with experimental results was conducted including flow pattern maps, heat transfer and pressure drop correlations. Some of the correlations, proposed for conventional channels and microchannels, showed good agreement with the present results. [ABSTRACT FROM AUTHOR]

Flow boiling characteristics of de-ionized water were tested experimentally in a rectangular copper single microchannel of 1 mm width, 0.39 mm height and 62 mm length. De-ionized water was supplied to the microchannel at constant inlet temperature (89°C) and constant inlet pressure (115 kPa). The mass flux ranged from 200 to 800 kg/m²s and the heat flux from 56 to 865 kW/m². The heat transfer rate data are presented as plots of local heat transfer coefficient versus vapour quality and distance along the channel. Flow visualization was also conducted using a high-speed, high-resolution camera. The results indicate that unstable flow boiling occurred starting at boiling incipience for all mass flux values. The local heat transfer coefficient depends on heat flux only at very low heat and mass fluxes. At high mass flux, there is no heat flux effect with little dependence on vapour quality after the entry region. The mass flux effect was more complex. [ABSTRACT FROM AUTHOR]

• Surface characteristics of copper and aluminium microchannels are different. • The flow boiling patterns observed in Cu and Al microchannels were similar, i.e. bubbly, slug, churn and annular flow. • Flow reversal was observed. • Flow boiling heat transfer coefficient in Al microchannels was higher by 12% average. • Pressure drop in Al channels was higher but overall pumping power is low in both Cu and Al microchannels. • Overall thermal performance of Cu and Al microchannels was similar. • Flow boiling in both Al and Cu microchannels is a viable option in cooling high heat flux devices. The substrate material and active side characteristics can affect the bubble ebullition cycle and consequently the heat transfer rate and pressure drop in microchannel evaporators. This paper presents an experimental study on flow boiling patterns, heat transfer rates and pressure drop in multi-microchannels evaporators made of copper and aluminium. HFE-7100 was used as the test fluid at atmospheric pressure, 5 K inlet sub-cooling, mass flux of 50-250 kg/m2s and wall heat flux up to 174 kW/m2. All heat sinks were made with channel width 0.46 mm, channel height 0.46 mm, giving a 0.46 mm channel hydraulic diameter. The heat sink base area was 25 mm in length and 20 mm in width. The experimental results showed that similar flow patterns were visualised for copper and aluminium namely bubbly, slug, churn and annular flow. The heat transfer coefficient in the aluminium heat sink was 12% (average value) higher than that found in the copper heat sink. The measured pressure drop in the aluminium heat sink was 28% (average value) higher compared to the copper heat sink. However, the additional pumping power required to move the fluid through the heat exchanger is small for this factor to be significant. The SEM images of the surface revealed that the number of cavities (possible nucleation sites) was higher in the aluminium surface with clear-cutting marks compared to the copper surface. This may explain the different pressure drop and heat transfer behaviour. The results of the present study indicate that aluminium heat sinks can offer comparable thermal performance to that of copper heat sinks and can also be recommended for cooling high heat flux systems. [ABSTRACT FROM AUTHOR]

Abstract: There is a large discrepancy in the open literature about the comparative performance of the existing macro and microscale heat transfer models and correlations when applied to small/micro flow boiling systems. This paper presents a detailed comparison of the flow boiling heat transfer coefficient for R134a in stainless steel micro tubes with 21 macro and microscale correlations and models. The experimental database that was used in the comparison includes the data for 1.1 and 0.52mm diameter tubes, mass flux range of 100–500kg/m2 s and system pressure range 6–10bar obtained in the course of this study. The effect of the evaporator heated length on the comparative performance of the correlations and models was investigated using three different lengths of the 1.1mm diameter tube (L =150, 300 and 450mm). This comparative study demonstrated that none of the assessed models and correlations could predict the experimental data with a reasonable accuracy. Also, the predictability of most correlations becomes worse as the heated length increases. This may contribute in explaining the discrepancy in the comparative performance of the correlations from one study to another. A new correlation is proposed in the present study based on the superposition model of Chen. The database used in developing the correlation consists of 5152 data points including the current experimental data and data obtained previously with the same test rig, fluid and methodology for tubes of diameter 4.26, 2.88, 2.01mm. The new correlation predicted 92% of the data within the ±30% error bands with a MAE value of 14.3%. [Copyright &y& Elsevier]

• Bubble size in nucleate boiling region increased with decreasing aspect ratio. • Wall heat transfer rates increases at high aspect ratio. • Base heat flux increases with decreasing aspect ratio. • The pressure drop increased with increasing aspect ratio. • Effect of aspect ratio on pumping power was not significant. The effect of channel aspect ratio on flow boiling characteristics (flow patterns, heat transfer and pressure drop) of HFE-7100 in copper multi-microchannel heat sinks was investigated experimentally. Three heat sinks with base area 500 mm2, channel hydraulic diameter 0.46 mm and channel aspect ratio (ratio of channel width to channel height) of 0.5, 1.0 and 2.0 were tested. The average surface roughness of the channel bottom surface was nearly the same in the three heat sinks and the measured values were 0.271, 0.286 and 0.304 µm. The local heat transfer rates were determined simultaneously with flow visualisation at mass flux ranging from 50 to 250 kg/m2s, wall heat flux from 9.6 to 191.6 kW/m2, system pressure of 1 bar and low inlet sub-cooling of 5 K. The results showed that, when the channel aspect ratio increased, the heat transfer coefficient increased, while the flow boiling pressure drop decreased. However, the heat transfer rate calculated using the heat sink base area was higher in the heat sink with the smallest channel aspect ratio, indicating an enhancement due to the largest surface area. [ABSTRACT FROM AUTHOR]