تجزیه و تحلیل عملکرد سه بعدی مبدل های حرارتی لوله دشت باله در رژیم انتقالی
|کد مقاله||سال انتشار||مقاله انگلیسی||ترجمه فارسی||تعداد کلمات|
|28050||2014||10 صفحه PDF||سفارش دهید||5275 کلمه|
Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Applied Thermal Engineering, Volume 50, Issue 1, 10 January 2013, Pages 445–454
Three-dimensional CFD simulations are carried out to investigate heat transfer and fluid flow characteristics of a four-row plain fin-and-tube heat exchanger using the Commercial Computational Fluid Dynamics Code ANSYS CFX 12.0. Heat transfer and pressure drop characteristics of the heat exchanger are investigated for Reynolds numbers ranging from 400 to 2000. Fluid flow and heat transfer are simulated and results compared using both laminar and turbulent flow models (k-ω) with steady and incompressible fluid flow. Model validation is carried out by comparing the simulated case friction factor (f) and Colburn factor (j) with the experimental data of Wang et al. . Reasonable agreement is found between the simulations and experimental data. In this study the effect of geometrical parameters such as fin pitch, longitudinal pitch and transverse pitch of tube spacing are studied. Results are presented in the form of friction factor (f) and Colburn factor (j). For both laminar and transitional flow conditions heat transfer and friction factor decrease with the increase of longitudinal and transverse pitches of tube spacing whereas they increase with fin pitches for both in-line and staggered configurations. Efficiency index increases with the increase of longitudinal and transverse pitches of tube spacing but decreases with increase of fin pitches. For a particular Reynolds number, the efficiency index is higher in in-line arrangement than the staggered case.
Plate fin-and-tube heat exchangers of plain fin pattern are commonly used in the process and HVAC&R (heating, ventilating, air conditioning, and refrigeration) industries. They are compact and light weight. Plain fins are used on the outer surface of the round tubes of staggered or in-lined arrangement to improve the heat transfer coefficient on the gas side in the heat exchangers. The heat transfer between the gas, fins and the tube surfaces is determined by the flow structure. The governing thermal resistance for an air-cooled heat exchanger is usually on the air side which may account for 85% or more of the total resistance . As a result, to effectively improve the thermal performance and to significantly reduce the size and weight of air cooled heat exchangers, the use of enhanced surfaces is very popular in air cooled heat exchangers. The plain plate fin configuration is the most popular fin pattern, due to its simplicity, durability and versatility in application. During the past few decades many efforts have been devoted to heat transfer and friction characteristics of plain fin-and tube heat exchangers ,  and . Also a number of correlations are developed by the researchers ,  and . There are also a number of numerical studies for plain fin-and-tube heat exchangers. Most of the earlier researchers used 2-D and laminar flow conditions in their numerical calculations . These authors noted that 2-D flow field studies cannot sufficiently predict heat transfer between the fluid and the fin; hence their simulations have limited application. Few researchers have reported 3-D modelling for plain-fin configuration in their numerical studies , ,  and . The experimental studies have shown that the flow range for the plain-fin configurations extends from laminar to the transitional range . So it is necessary to determine the flow structure for the plain fin configuration for both laminar and transitional flow regions numerically. Panse  has done a comparative numerical analysis of the three turbulence models namely κ-ɛ model, RNG κ-ɛ model and the k-ω model. In the present study, 3D CFD simulations have been carried out to investigate the performance for the plain fin configuration for both laminar and transitional flow regions because, most of the early researchers have considered 2D modelling in their numerical analysis. A researcher reported 2-D numerical results along with experimental data for the influence of fin spacing on the heat transfer and pressure drop, Kundu et al. . These researchers were limited, because they were trying for solutions for a three dimensional problem with two dimensional approach. The authors noted in their study that the two dimensional field studies cannot sufficiently predict heat transfer between the fluid and the fin. The approximations used by the authors were to avoid the side wall effects. Zdravistch et al.  used different boundary conditions for his study in the two dimensional flow field. The author specifically mentioned the importance of three dimensional simulations in case of side wall effects, which is exactly the case with heat exchangers which use fin and tube banks. It is understandable from the literature that for the flow channel consists of a complicated structure, numerical studies with two-dimensional approach is not sufficient. So for the present study of similar type of geometry and boundary condition, 2D model will not be the appropriate as side wall has significant effect on the performance of heat exchanger performance.
نتیجه گیری انگلیسی
The numerical investigation of heat transfer and pressure drop for the plain fin-and-tube heat exchanger for laminar (200 ≤ ReH ≤ 1200) and transitional (1300 ≤ ReH ≤ 2000) flow regimes was carried out considering in-lined and staggered arrangements using the Commercial CFD Code ANSYS CFX 12.0. The present paper describes an effort to reveal the effects of Reynolds number, fin pitch and tube pitches on the overall Heat transfer and friction factor for plate fin-and-tube heat exchangers with large tube diameter. 3D simulations with turbulent model are conducted to compare the heat transfer and friction factor characteristics of fin-and-tube heat exchangers with different geometric parameters. Tube arrangements play a vital role in the thermal and hydraulic characteristics of fin-and-tube heat exchangers. For both laminar and transitional flows, staggered tube arrangement produces better flow mixing and thus provides higher heat transfer and pressure drop characteristics than the in-lined arrangement. The increase in the longitudinal pitch (Ll) and transverse pitch (Lt) causes a decrease in the heat transfer and pressure drop performance as the flow becomes free and less compact. As the pressure drop decrease is more significant than heat transfer, the efficiency goes up with the increase in fin pitch. However, decrease in the fin pitch shows opposite performance because the flow becomes more streamlined. It affects the heat transfer performance as well as pressure drop characteristics.