• Energy Research

Abstract This paper applies the computational fluid dynamics (CFD) simulation to study the condensate two-phase thermofluid characteristics of refrigerant HFO-1234yf flowing through horizontal straight and convergent passages for guiding the design of minitube heat exchangers. The theoretical analysis employs the volume-of-fluid (VOF) method to model the progression of film condensation process for determining the distributions of velocity, pressure, temperature, vapor volume fraction and film thickness for liquid-vapor interfacial flows. The Lee model is formulated as a user defined function to treat the phase change effect at the interface. The predictions of heat transfer coefficients and pressure drops agree reasonably well with the measured data from the referencing literature at the mass and heat fluxes of 200–800 kg/m2 s and 9.9–24.3 kW/m2 for validation of the computational model. The simulated results are presented to capture the complex two-phase flow behaviors during the film condensation progression, including the formation of interfacial ripples of annular films, wave-shaped films with elongated necking regions, shedding of liquid ligaments and breakup of these ligaments entrained into the vapor core. The VOF calculations also estimate the average heat transfer coefficient and pressure drop up to 11.3 kW/m2 K and 55.1 kPa at a mass flux of 800 kg/m2 s for the straight tube, showing similar thermal performance to refrigerant HFC-134a reported by open literature. The convergent passage essentially raises the average heat transfer coefficient and pressure drop because of more severe shear stresses over the film surface. The correlations of heat transfer coefficient and pressure drop are compared with CFD predictions for straight and convergent tubes. The performance assessment is thus conducted using the validated correlations to guide the redesign of minitube heat exchangers with HFO1234yf as an alternative refrigerant in various energy systems. Two design changes are presented using HFO1234yf refrigerant to enlarge the total surface area up to 13.49% in straight tubes or broaden the cooling surface area of only 6.4% in convergent tubes for matching the performance of R134a.

Abstract In the present study, a multiple jet-cooling device for electronic components was investigated, using FC-72 as the working fluid. The nozzle plate, located 5 mm above the 12 × 12 mm2 test surface, had 5 or 9 pores of 0.24 mm in diameter. The test surfaces included a smooth surface, two pin-finned surfaces and two straight-finned surfaces of 400 or 800 μm fin height, 200 or 400 μm fin thickness and gap width. The results showed that the heat transfer performance increased with increasing flow rate or increasing surface area enhancement ratio. The pin-finned surface of 800 μm fin height, 200 μm fin thickness and gap width yielded the best performance, which was about 250% greater than the smooth surface at 150 ml/min. Correlations of two-phase multiple jets, cooling in free and submerged states, are proposed based on the data at 50 °C saturation temperature, in the range of Re = 1655–8960, Bo = 0.024–0.389, area enhancement ratio = 1.0–5.32, jet spacing-diameter ratio (S/d) = 13.7 and 20.6. The root mean square deviation of the prediction is 11.96% for the free jet data, and 9.08% for the submerged jet data. Thermal resistance of the best surface varied between 0.1 and 0.13 K/W at 150 ml/min flow rate in the range of 60–120 W heat input.

This study aims to investigate the water spray uniformity and collection ratio of sprinkler in an evaporative condenser of a water chiller. Experiments of water droplet distribution are conducted with 50 water collectors during the tests. Three different combinations of nozzle opening length and width were tested with the flow rates varied at 135 LPM and 176.4 LPM. Measured results show that the cross-sectional area of nozzle opening and flow rate significantly affect the water spray uniformity. In this work, at high flow rate, the Nozzle 2 with opening of 4 cm in length and 1 cm in width has better water spray uniformity compared to the nozzle 1 with opening of 4 cm in length and 0.7 cm in width. On the other hand, at low flow rate, the Nozzle 1 provides better impacting effect with the nozzle spacing of 17 cm, yet the Nozzle 2 performed better with the nozzle spacing of 15 cm. The latter case, with the smaller nozzle spacing and bigger nozzle opening size, led to a shorter impact distance of the spraying flow from two facing nozzles. Subsequently the spattering of water droplets was more pronounced, and distributed more uniformly. Keywords: Water spray uniformity, Collection ratio of sprinkler, Evaporative condenser, Nozzle opening