مقاله انگلیسی رایگان در مورد تحلیل عددی صفحه پره ای متراکم مبدل حرارتی – امرالد 2017

Depending on the application, various types of augmented heat transfer surfaces such as wavy fins, offset strip fins, louvered fins, plain and perforated fins are used in aerospace applications. The surface geometries of wavy and OSF fins are described by the fin height (h), transverse spacing (s) and thickness (t). Interrupted flow length of the offset strip fin is described by offset strip/fin length (l), and that of the wavy fin, by the pitch of the wave (L). Thermo-hydraulic design of a compact heat exchanger is strongly dependent upon the performance of heat transfer surfaces (in terms of Colburn factor j and Fanning friction factor f vs. Reynolds number Re characteristics). We focus here on offset strip fins, wavy fins, plain and perforated fins. The orientation of inlet and outlet headers plays a major role in performance especially in aerospace applications, where the orientation of headers and nozzles are not straight and uniform due to space limitations. The accurate prediction of the thermal performance of a compact heat exchanger in the design stage is highly desirable for aerospace applications. The longitudinal heat conduction (LHC) through the heat exchanger wall structure in the direction of fluid flows has the effect of decreasing the exchanger performance for a specified NTU, and this reduction may be quite serious in exchangers with short flow length designed for high effectiveness (> 80%) [1]. These effects have been well recognized and the numerical data are available in [2,3] for periodic-flow heat exchangers and in [4-6] for the direct transfer type heat exchangers. The flow maldistribution effects have been well recognized for heat exchangers. The flow non uniformity through the exchanger is generally associated with improper exchanger entrance configuration, due to poor header design and imperfect passage-to-passage flow distribution in a highly compact heat exchanger caused by various manufacturing tolerances. The flow non uniformity (FN) effects have been well recognized and presented for heat exchangers[7-14]. Ranganayakulu et al.[7] carried out the Finite Element analysis for effects of FN on cross flow Compact Heat Exchanger (CHE). Chiou [8-11] carried out the FN effects using Finite Difference Method for various types of heat exchangers, such as cross flow heat exchanger [8], automobile airconditioning condenser [9] and evaporator [10] and FN effects on both cold and hot fluids of cross flow heat exchanger [11]. Kranc [12] studied the effect of non uniform water distribution on cooling tower performance. Similarly, the fluid inlet temperature non uniformity (TN) effects have also been investigated for cross flow heat exchangers [15,16]. In actual practice, heat exchangers may be subjected to wall LHC, inlet FN and TN together. Literature on the investigation of combined effects of LHC, TN and FN for a cross flow plate-fin heat exchanger is limited [15,16]. Moreover, all the previous works [8-12] were limited to specific types of non-uniform flow models and can not be interpolated or extrapolated for other types of flow maldistributions. Also, Chiou [13] analysed the effects of LHC and FN on compact heat exchangers. Zhang et al. [17] have investigated the flow non uniformity in a plate-fin heat exchanger by a CFD software. Based on the investigation, two modified headers with a two-stage-distributing structure are proposed to reduce the flow non uniformity. Ranganayakulu et al. [18] studied the effects of the fluid flow non uniformity due to the improper header/nozzle configuration with the CFD tool for a typical stainless steel compact plate-fin heat exchanger. Wen et al. [19] investigated flow characteristics of flow field in the entrance of a plate-fin exchanger by means of Particle Image Velocimetry (PIV). Based on experiments, they suggested that punched baffle could effectively improve fluid flow distribution in the header.