Search

The porous layer subjected to air jet is set above fluid passing through heating surface to control its thickness and avoid drying up, as well as ultrasound is introduced at upstream to enhance convection in flowing channel, in which heat flux can be dispersed in convection between fluid and heating surface as well as evaporation on the outside surface of porous cover to the outside. Local thermal equilibrium (LTE) occurs in porous cover, and the effect of sound pressure from ultrasound on liquid fluid is introduced according to acoustic theory. Brinkman–Forchheimer extended Darcy equations and k-ε turbulent model together with the above formulas are employed to describe the heat transfer in fluid covered by porous media subjected to ultrasound and air jet impingement. The effects of ultrasound power and frequency, fluid layer thickness as well as porosity and particle size in porous cover on heat transfer are investigated. The performance evaluation criteria (PEC) is defined to evaluate thermal characteristics in the cases with ultrasound than those without. The 35.7% rise ratio of heat transfer coefficient in the mode with ultrasound power of 50 W happens than that without. The higher ultrasound power and lower frequency of ultrasound cause lower heating surface temperature and larger heat transfer coefficient. The simulations agree with available published experiment data. All results of this study can be considered into utilization of ultrasound and porous layer to enhance heat transfer in flowing channel. • Low frequency and high power of ultrasound benefit thermal-hydraulic behaviors. • More evaporation and convection in the cases with thicker fluid under ultrasound. • Large porosity and particle in porous cover favor thermal behaviors under ultrasound. [ABSTRACT FROM AUTHOR]

Convective heat transfer to an impinging air jet is known to yield high local and area-average Nusselt numbers. We simulate this heat transfer over a wide range of study parameters, including Reynolds number and mist mass fraction, and three jet shapes: circular, half circuit, and quarter circuit. Simulations show that when compared with air only, mist provides higher heat-transfer enhancements for the first two shapes but is insignificant for the third. The area-average Nusselt number is higher by ∼100% for the half-circuit jet than for the circular jet for both single-phase air and mist. With 0.5% mist, the area-average Nusselt number of the circular jet is enhanced by 41% at Reynolds number 10000. [ABSTRACT FROM AUTHOR]

The jet impingement heat transfer is a well-established technique for obtaining high heat transfer rates for many cooling and heating applications. Impingement heat transfer methods are widely used in domestic and industrial appliances. The impinging isothermal gas jets are used for both heating and cooling. However, premixed flame jets are used for heating the target surface. Present study is an attempt to compare the heat transfer characteristics of the isothermal air jets and premixed flame jets. The isothermal air jet having jet temperature of 30 °C (cold jet or ambient temperature jet), 100 °C (hot jet), and premixed methane–air flame jet (stoichiometric mixture) with Reynolds numbers of 500, 750, 1000, 1250 and 1500 and nozzle or burner to plate spacings of 2 d , 4 d and 6 d are experimentally investigated for heat transfer characterisation. Thin metal foil technique is used for characterising isothermal air jets. For a fixed jet Reynolds number, the Nusselt number of isothermal air jet is found independent of the temperature difference between jet and surrounding fluid. A steady state technique is proposed for characterisation of premixed flame jets. The outcome of the present study reveals that the heat transfer characteristics of isothermal air jet and the premixed flame jet are almost same. The higher thermal entrainment in premixed flame jet in comparison with isothermal air jet results in a lower effectiveness. The present heat transfer data can be directly utilised for many practical applications. [ABSTRACT FROM AUTHOR]