ENHANCEMENT OF HEAT AND MASS TRANSFER USING JET REATTACHMENT NOZZLES OVER STATIONARY AND MOVING SURFACES - EXPERIMENTAL AND NUMERICAL STUDY

Farzad, Milad

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ENHANCEMENT OF HEAT AND MASS TRANSFER USING JET REATTACHMENT NOZZLES OVER STATIONARY AND MOVING SURFACES - EXPERIMENTAL AND NUMERICAL STUDY

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Slot jet reattachment (SJR) nozzle is developed in an attempt to enhance heat and mass transfer characteristics while effectively controlling the impingement surface force exerted by the jet flow. In the SJR nozzle, the jet is directed outward from the nozzle exit and it then reattaches on an adjacent surface in its vicinity. The turbulent mixing occurs at the boundaries of the free stream induces secondary flow by mass entrainment and causes the flow to reattach the surface in the form of an oval reattachment at close nozzle to surface spacing [1].Since drying is an energy-extensive process, many efforts have been made to come up with an energy-efficient technique while food product quality is kept at a desirable level. Due to the high transport characteristics provided by impinging jet nozzles, utilizing such nozzles is the most common technique in drying various types of pulp paper and food products. However, conventional perforated plates, in-line jet (ILJ), and slot jet (SJ) nozzles waste a great deal of energy and degrade the food products during this rapid drying process. Given the lack of efficiency of traditional nozzles, innovative impinging jet nozzles are a significantly better replacement to improve efficiency and time in the drying process, while maintaining an appropriate level of quality in the food products.In this research, the drying characteristics of Radial Jet Reattachment (RJR) and Slot Jet Reattachment (SJR) nozzles, with exit angles of +20° and +45°, were experimentally investigated in drying hand-sheet paper and food products on a stationary and a moving surface. Dry basis moisture content of the samples using these reattachment nozzles were compared to those of traditional nozzles, based on three criteria: identical air mass flow rate, identical fluid flow power, and identical exerted peak surface pressure. To appraise the performance of each nozzle, the corresponding local heat transfer coefficients generated by reattachment and traditional nozzles for all the tests were measured. Subsequently, the average heat transfer coefficients of reattachment nozzles were calculated and compared against those of traditional nozzles. More-over, the kinetics of color changes and structure analysis of apple slices, due to the drying by SJ and SJR+20° nozzles were studied. The experimental results revealed that the RJR+45° andSJR+20° nozzles yield significantly higher drying rates, as well as significant improvement in average heat transfer coefficients, compared to those of ILJ and SJ nozzles, respectively, on both stationary and moving surfaces. Besides, impingement drying with SJR+20° nozzle produced dried samples with less color deterioration and fewer nutritional losses compared to drying with the SJ nozzle. Therefore, jet reattachment nozzles can provide faster drying rates and keep the nutritional values at a desirable level.Furthermore, This work numerically investigates the convective air-drying characteristics of anSJR nozzle and compares it to those of a common SJ nozzle for a moist porous medium. The comparison is made under identical air mass flow rate criterion. The results are presented for stationary and moving reattachment surface under a laminar flow condition. The governing transport equations within a moist porous medium are coupled with the governing equations of the resultant air flow. The results indicate a much higher drying rates with SJR nozzle com-pared to SJ nozzle. Moreover, the effects of porous medium properties on the drying time were studied.

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