New empirical correlation models are constructed to characterise heat transfer associated with spray evaporative cooling of vibrating surfaces -a process involving complex two-phase physics well beyond current numerical simulation capabilities. The proposed correlation models, which account for dynamic, rather than just static surface conditions as in existing models, are constructed using dimensional analysis involving the Generalized Buckingham Π-Theorem. Experimentally-measured spray evaporative cooling data is used to fit the model using the Vibrational Reynolds number and a dimensionless acceleration number which better correlate the influence of surface frequency and amplitude in the nucleate boiling regime. Different coolant flow-rates through a full-cone spray nozzle are used to cool a flat circular test-piece acting as a horizontal surface. The test-piece surface is excited by a shaker through a range of low and high vibration frequencies and amplitudes. The results show that surface dynamic effects certainly influence nucleate boiling, but they also show that surface vibration does not have the same effect for all excess temperatures -dynamic effects can either increase or decrease heat transfer depending on the heat transfer mechanism. These new models are important for thermal management in several areas, particularly involving batteries, power electronics, and electrical machines in automotive and aerospace applications.
In the present study, condensation heat transfer and frictional pressure drops of refrigerant R-600a (iso-butane) inside a helically dimpled tube and a plain tube of internal diameter 8.3 mm were measured and analyzed. All tests were performed at different vapor qualities up to 0.82 and average saturation temperatures ranging between 38 and 42℃. Refrigerant mass fluxes varied in the range of 114-368 kg/m 2 s. The inner surface of the helically dimpled tube has been designed and reshaped through three-dimensional material surface modifications consists of both shallow and deep protrusions which are placed evenly in helical directions on the tube wall. The experimental results show that the heat transfer coefficients of the dimpled tube are 1.2-2 times of those in smooth tube with a pressure drop penalty just ranging between 58% and 195%. The highest heat transfer coefficient is occurred at vapor quality of 0.53 and mass flow rate of 368 kg/m 2 s. On the other hand, the maximum increase of pressure drop takes place at vapor quality of 0.55 and mass flow rate of 368 kg/m 2 s. Nomenclature ݉ሶ mass flow rate (kg/s) A C p surface area (m ଶ ) specific heat (kJ/kg k)
Maziar (2019) An experimental study on condensation heat transfer characteristics of R-600a in tubes with coiled wire inserts. Applied Thermal Engineering.
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