تجزیه و تحلیل عملکرد حرارتی بلوک های جامد و سوراخ شده متصل شده روی یک سطح صاف در جریان داکت

The thermal performances of solid and perforated rectangular blocks attached on a flat surface in a rectangular duct were determined with respect to the heat transfer from the same plate without blocks. The data used in the performance analyses were obtained experimentally for varying flow and geometrical conditions. It was found that the solid blocks generated a net energy loss despite significantly enhanced heat transfer due to the increased heat transfer surface area. When the blocks were perforated, the loss in the net energy was recovered and depending on the geometrical and flow conditions, a net gain in energy, up to 20%, was achieved. For both the solid and the perforated blocks, increases in Reynolds number led to decreases in the performance.

The increasing necessity for saving energy and material imposed by diminishing world resources has prompted the development of more effective heat transfer equipment for more efficient use of energy and material. In many industrial systems, heat must be transferred either to input energy into the system or to remove the energy produced in the system. Considering the rapid increase in energy demand worldwide, both reducing energy loss due to ineffective use and enhancement of the energy transfer in the form of heat has become an increasingly important task for design and operation engineers. In recent years, many techniques have been proposed for the enhancement of heat transfer. These can be classified into two main groups. Those which do not require an additional power source may be named as passive techniques and those with additional external power requirements active techniques [1] and [2]. In the case of passive techniques, attachments of heat transfer promoters of different shapes and geometries, such as fins, ribs and blocks, on the heat transfer surface have been widely exploited. Heat transfer is enhanced by the increase in the surface area and also by the turbulence or mixing generated due to the attachments. Enhancement in convective transfer rates is obtained at the expense of the energy dissipated by extra friction caused by non-smooth surfaces and insertions. In the case of active techniques, in addition to frictional energy dissipation, the external power input must also be taken into account. 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 the Stanton number (St) to the friction factor (f), St/f, obtained with and without the heat transfer promoter [3] and [4]. Zimparow and Vuchanow [5] developed a performance evaluation criterion based on an entropy production theorem. Sano and Usui [6] and [7] suggested that for a heat exchanger, the heat transfer coefficient could be correlated as a function of the energy dissipation per unit mass of fluid (ϵ). Thus, they were able to evaluate a simple performance evaluation criterion by comparing the heat transfer coefficients for a constant ϵ. Another performance evaluation criterion is the comparison of the heat transfer coefficient per unit pumping power at constant surface [8] and [9]. However, it is difficult to decide on the proper criterion for heat transfer enhancement because so many factors must be considered, i.e. operational and maintenance cost, safety, reliability, surface area and pumping power. It also depends on the design objectives, such as: (1) reduced heat transfer surface for equal pumping power and heat duty; (2) increased UA product for equal pumping power and fixed total length of exchanger tubing; and (3) reduced pumping power for equal heat duty and total length of exchanger tubing. Enhancement of the convective heat transfer from a flat surface due to rectangular cross-sectional blocks in a parallel flow in a rectangular duct have been investigated experimentally [10]. For the same flow conditions, in another study [11], we reported the effect of perforations of different geometries made in the blocks, both on the turbulent heat transfer and pressure drop. By the present paper, it is aimed to analyse the thermal performances of these heat transfer promoters with respect to their heat transfer enhancement efficiencies for a constant pumping power.

The thermal performances of solid and perforated rectangular blocks attached on a flat surface in a rectangular duct were determined with respect to heat transfer from the same plate without blocks by using the heat transfer enhancement efficiency as a performance criterion. The results indicated that: • The solid blocks enhance the heat transfer from the plate significantly as a result of the increased heat transfer surface area. However, since they yield higher pressure drops in the flow and lead to poorer flow contact with the base plate, they generate a net energy loss. • When the blocks are perforated, the net energy loss is recovered, and depending on the geometrical and flow conditions, a net energy gain (as much as 20%) is achieved. • For both the solid and the perforated blocks, increases in Re lead to decreases in the performance. • For the perforated blocks, the higher the perforation diameter, perforated area open area ratio, and inclination of the perforation holes towards the surface of the plate, the better their heat transfer enhancement performance. • Increases in the number of blocks leads to a better performance for the perforated blocks, but for the solid blocks, it makes no difference.