Mohamed M. Mahmoud

Flow Boiling Pressure Drop of R134a in Microdiameter Tubes: Experimental Results and Assessment of Correlations

The experimental results of two-phase flow boiling pressure drop of R134a in vertical microdiameter stainless steel tubes are presented in this paper. The tests were conducted using four tubes: one tube with an inner diameter of 0.52 mm and 100 mm heated length and three tubes with an inner diameter of 1.1 mm and different heated lengths (150, 300, and 450 mm). Other experimental conditions include mass flux range of 200–500 kg/m2-s, system pressure range of 6–10 bar, inlet subcooling value of about 5 K, and heat flux range of 1–140 kW/m2. The results indicated that the total measured two-phase pressure drop increases with increasing mass flux, heat flux (exit quality) and decreasing system pressure and tube inner diameter. The test section heated length was found to have a significant effect on the measured pressure drop per metre length. The total measured two-phase pressure drop results were also compared with eighteen macro- and microscale models and correlations.

One-dimensional semimechanistic model for flow boiling pressure drop in small to micro passages

Accurate predictions of two-phase pressure drop in small to micro-diameter passages are necessary for the design of compact and ultra-compact heat exchangers, which find wide application in process and refrigeration industries and in the cooling of electronics. A semimechanistic model of boiling two-phase pressure drop in the confined bubble regime is formulated, following the three-zone approach for heat transfer. The total pressure drop is calculated by time-averaging the pressure drops for single-phase liquid, elongated bubble with a thin liquid film, and single-phase vapor. The model results were compared with experimental data collected for a wide range of tube diameters (4.26, 2.88, 2.02, 1.1, and 0.52 mm) for R134a at pressures of 6–12 bar. In this models present form, its predictions are close to those of the homogeneous flow model but it provides a platform for further development.

Flow Boiling of R134a and R245fa in a 1.1 mm Diameter Tube

New refrigerants are required for cooling systems due to the fact that refrigerants like R134a are about to be phased out. This paper presents a comparison between the flow boiling heat transfer and pressure drop results of refrigerants R245fa and R134a. The experiments with R245fa were conducted in a vertical cold drawn stainless steel tube with an inner diameter of 1.1 mm and heated length of 150 mm. Experimental conditions include: mass flux range 100–400 kg/m2s, heat flux range 10–60 kW/m2, pressures of 8 and 10 bar and 1.9 and 2.5 bar for R134a and R245fa corresponding to saturated temperatures 31 °C and 39 °C and exit vapour quality range 0–0.95. The data for R134a were obtained earlier using the same experimental facility at the same experimental conditions and with the same test tube. The results demonstrated that refrigerant properties have a significant effect on heat transfer and pressure drop. The pressure drop of R245fa is higher by up to 300% compared to that of R134a at similar conditions. In addition, the effect of mass flux and heat flux on the local flow boiling heat transfer coefficient was different. Heat transfer coefficients of R245fa showed a greater dependence on vapour quality. The agreement with past heat transfer correlations is better with R134a than with R245fa.Copyright

A Study of Discrepancies in Flow Boiling Results in Small to Micro Diameter Metallic Tubes

There is a disagreement in the reports on flow boiling heat transfer on the dependence of the local heat transfer coefficient on local vapour quality, mass and heat flux and system pressure. As a result, various conclusions were reported about the dominant heat transfer mechanism(s) in small to micro diameter tubes. Yet, the reasons behind this large disagreement are not clear. The current study investigated experimentally two important parameters that may contribute in explaining the scatter in the published heat transfer results. The first parameter is the tube inner surface characteristics whereas the second is the length of the heated section. The surface effect was experimentally investigated through examining two stainless steel tubes manufactured by two different methods. The first tube is a seamless cold drawn tube whilst the second is a welded tube. The two tubes have similar design and dimensions and were investigated at 8 bar system pressure and 300 kg/m2 s mass flux. The inner surface of the two tubes was examined using a scanning electron microscope (SEM) and was found to be completely different. The heat transfer results demonstrated that the trend of the local heat transfer coefficient versus local vapour quality in the seamless cold drawn tube is completely different from that in the welded tube. Three heated lengths were investigated for a seamless cold drawn tube with an inner diameter of 1.1 mm over a wide range of experimental conditions; mass flux range of 200–500 kg/m2 s, system pressure of 6–10 bar, inlet sub-cooling value of about 5K and exit quality up to about 0.95. The results indicated that the heated length strongly influences the magnitude as well as the local behaviour of the heat transfer coefficient. There is a progression from nucleate boiling to convective boiling as the heated length increases. The variation in the heat transfer coefficient due to differences in the heated length may also influence the performance of the existing micro scale heat transfer correlations. The flow patterns observed at the exit of each test section are also presented.Copyright