انتقال حرارت و ارتباط اصطکاک و تجزیه و تحلیل عملکرد حرارتی برای یک سطح پره دار

In the present work, the heat transfer and friction loss characteristics were investigated experimentally, employing a finned heating surface kept at a constant temperature of 45°C in a rectangular channel through which air was passed as working fluid. The position of the cylindrical fins attached on the surface was arranged either in-line or staggered. The parameters for the study were chosen as Reynolds number (3700–30 000), depending on hydraulic diameter, the distance between fins in the flow direction (Sy/D=1.96–4.41) and fin arrangement. The variation of Nusselt number with these parameters was determined and presented graphically. For both fin arrangements, it was found that increasing Reynolds number increased Nusselt number, and maximum heat transfer occurred at Sy/D=2.94. Correlations for Nusselt number and friction factor were developed for both fin arrangements and smooth channel, and the thermal performances of the arrangements were also determined and compared with respect to heat transfer from the same surface without fins. With the staggered array, a heat transfer enhancement up to 33% at constant pumping power was achieved.

If the heat formed in a working machine is not removed at a sufficient rate, some problems, even breaking down, can take place in the machine due to over heating. Overcoming this sort of problem is only possible by a more effective heat transfer. Furthermore, considering that alternative energy resources have to be researched to replace the rapidly consumed present energy resources, the economic use of heat energy depends on the transfer of heat more efficiently. One of the ways of increasing the heat transfer rate is to attach fins on the heat transfer surface. This technique is widely used in applications, such as power stations, cooling engines, cooling systems, vapour cooling systems etc. Therefore, optimisation of the fin design is very important to reduce the size of the heat transfer equipment and, consequently, to use less material. Not only do the fins increase the heat transfer area, which causes an increase in the quantity of transferred heat, but they also increase the turbulence of the flowing fluid [1] and [2]. Jubran et al. [3] reported on an experimental investigation to study the effects of shroud clearance, optimum inter-fin spacing in both spanwise and streamwise directions and missing pin upon heat transfer, and they correlated their heat transfer data. In a study performed by Tahat et al. [4], in a channel with rectangular cross section equipped with needle fins, the effects of the distance between fins, the space above them and their in-line and staggered arrangement on heat transfer were investigated experimentally, and they found that the optimum distance between the fins was 2.5 fin diameters, both parallel and spanwise to the flow direction. Hwang and Lui [5] performed an experimental work to make a comparison of end wall heat transfer and pressure drop characteristics between the pin fin trapezoidal duct with straight and lateral outlet flow, using a transient liquid crystal method to measure the local heat transfer coefficient on the end wall surfaces. Vanfossen [6] conducted an experimental study to compare the overall heat transfer coefficients in rectangular ducts with staggered arrays of short and long pin fins. The results showed that the short pin fins performed better overall heat transfer than long pin fins. There are some studies about heat and mass transfer from surfaces with cylindrical fins using a naphthaline sublimation technique [7] and [8]. Enhancement in convective transfer rates is obtained at the expense of the energy dissipated by the extra friction caused by non-smooth surfaces and insertions. Therefore, for practical applications, a thermal performance analysis is worthwhile for evaluation of the net energy gain in the form of heat. The simplest way to evaluate the heat transfer enhancement performance of a given heat transfer promoter is to compare the ratio of Stanton number (St) to friction factor (f), St/f, obtained with and without the heat transfer promoters [9] and [10]. Another performance evaluation criterion is the comparison of the heat transfer coefficients at constant pumping power [11], [12] and [13]. The present work submits an experimental study on the heat transfer and friction loss characteristics of a surface with cylindrical fins in a channel having rectangular cross section with larger fin diameter and different channel geometry from the literature. The experiment was performed for two fin arrangements: in-line and staggered. Keeping constant the distance between fins in the spanwise direction to the flow, the effect of the distance between the fins in the flow direction on the heat transfer was investigated. In addition, heat transfer experiments without fins were also conducted for efficiency comparison. Furthermore, friction loss was determined by measuring pressure loss along the test section. The experimental parameters were chosen as Reynolds number, fin arrangement and fin distance in the flow direction.

In this study, heat transfer from a surface equipped with cylindrical fins was investigated. It was observed that although the heat transfer increased with increasing Re, the heat transfer enhancement decreased with Re. The staggered arrangement of the fins gave slightly higher enhancement than the in-line arrangement. However, as a consequence of the heat transfer enhancement, the pressure drop was higher for the staggered arrangement. The effect of distance between the fins on heat transfer for the staggered array were more pronounced than for the in-line array. The thermal performances of staggered and in-line fin arrays attached on a flat surface in a rectangular duct were determined with respect to heat transfer from the same surface without fins by using heat transfer enhancement efficiency as a performance criterion. The results indicated that: • The in-line and staggered fin arrays significantly enhanced the heat transfer from the surface as a result of the increased heat transfer surface area and turbulence but at the expense of higher pressure drops in the channel. • When the fins are in staggered array, the net energy loss is recovered, and depending on the fin distance, a net energy gain up to 33% was achieved. • For both the in-line and the staggered arrays, increases in Re led to decreases in the performance. • For the staggered array, a little better heat transfer enhancement performance was achieved than that for the in-line arrangement.