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The 6th Micro and Nano Flows Conference was held at the Georgia Institute of Technology, Atlanta, Georgia, USA, between 9-12 of September 2018, see http://mnf2018.com. The conference Local Organizi...

Pressure changes caused by the growth of confined bubbles during flow boiling in mini-/microchannels lead to transient flow reversal in the presence of inlet (upstream) compressibility. A one-dimensional (1-D) model is presented to study the effect of inlet resistance on maximum flow reversal distance, local pressure fluctuations for different initial upstream compressible volumes, channel dimension, locations of nucleation site, heat flux, and initial channel velocity for water and FC-72 at atmospheric pressure and R134a at 800 kPa. The two upstream compressibility models considered are condensable vapor in a subcooled boiling region and trapped noncondensable gas.

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.

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 model's present form, its predictions are close to those of the homogeneous flow model but it provides a platform for further development.

Experimental results for etectrohydrodynamic (EHD) effects on nucleate boiling at passively enhanced surfaces, such as Thermoexcel-HE and Gewa-T, are reported. The experiments were carried out using a single-tube test rig by applying a 0- to 30- kV dc potential to a dielectric liquid (Refrigerant-114). The EHD effect on the enhanced surfaces is compared with results of earlier experiments on smooth and Thermoexcel-C tubes and discussed. The results indicate that the boiling hysteresis at these passively enhanced surfaces can be eliminated and the nucleate boiling can be further enhanced and controlled by applying an electric field. An explanation of bubble initiation at low superheat by an electric field is offered in terms of the effect of the field on bubble critical radius.

A multiple regression analysis has been carried out on the monthly-mean global and diffuse solar irradiation data in Ibadan, Nigeria having the atmospheric specific humidity or water vapour content W at and the relative sunshine (n/N) as dependent variables, with the resulting equations: G/G 0 = 0.155 + 0.38(n/N) + 0.007 W at D/G 0 = 0.155 + 0.13(n/N) − 0.003 W at The standard error of estimate on the coefficients of the dependent variables is less than 0.03.

The effect of a radial d.c. electric field on nucleate boiling heat transfer was investigated experimentally using a single-tube shell/tube heat exchanger. A dielectric liquid (Freon R. 114) was used in the shell and the tube was heated by circulating water through it. It was found that the application of a sufficiently intense electric field to the boiling heat transfer surface resulted both in the elimination of boiling hysteresis and the enhancement of heat transfer for the range of superheat studied. An application of approximately 20 kV was more than sufficient to eliminate the hysteresis. It was also observed that there appeared to be a transition between two situations. At lower superheat there is an appreciable improvement in the heat transfer coefficient due to both initial (0 to 10 kV) and incremental (10 to 20 kV) voltage increases. At higher superheat, however, the greater improvement (about four times) is obtained with the initial voltage application