基于场协同理论的管翅式换热器翅片效率的数值模拟研究

本文对平直翅片管翅式换热器在考虑翅片效率和不考虑翅片效率2种情况下分别进行了层流状态下的流动和换热的三维数值模拟,研究了2种情况下Re数对换热量、Nu数及协同角的影响,并从场协同角度进行了分析。结果表明,Re数取200～1 700范围内的6个不同值,在不考虑翅片效率时,换热量分别增大了3.44%、4.76%、7.48%、12.68%、16.04%和19.56%。随着Re数的增大,翅片效率对模拟结果的影响也会增大,即在高Re数下不可忽视翅片效率对结果的影响。本文还从翅片效率的根本定义出发,得出了不同Re数下的翅片效率,为计算翅片效率提供了一个新的思路。     

Performance improvement of D-sorbitol PCM-based energy storage system with different fins

This paper is aimed at enhancing the heat exchange rate in the thermal energy storage system with dissimilar fins and encapsulation materials. Experimentation is done with d-sorbitol as phase change material with different encapsulating materials, copper, stainless steel (SS) and brass, in pin fin, annular fin and rectangular fin. The results are analysed for their thermal performance characteristics during both the charging and discharging processes. During charging, copper encapsulation showed a higher heat energy transfer rate around 2340?kJ. During discharging, a loss of 22% in heat extraction is seen in copper encapsulations, 6% and 19% in brass and SS encapsulations, respectively. These results show that the usage of annular fins is a valuable practice to increase the heat stored and recovered, in a latent heat storage system. The most cost-effective encapsulation is SS balls with annular fins as compared with copper and brass encapsulations.     

Performance evaluation of solar air heater having V-ribs with symmetrical gaps in a rectangular duct of solar air heater

An investigation has been carried out to evaluate the performance of a solar air heater duct roughened with V-ribs with symmetrical gaps to enhance heat transfer. The roughness produced for enhancing heat transfer from the absorber plate also enhances pressure drop due to increased friction. Heat transfer enhancement of the absorber plate has been experimentally determined by using V-ribs with symmetrical gaps. The duct has a relative roughness height (e/D) of 0.0433, a relative roughness pitch (P/e) of 10 and an angle of attack (α) of 60°. Relative gap width (g/e) and number of gaps (N_g) were varied in the range of 1–5. The maximum enhancement in the Nusselt number and friction factor is observed to be 2.59 and 2.87 times, respectively, as compared to the smooth duct. The relative gap width of 4 and the number of gaps of 3 show the maximum value of thermohydraulic performance parameter.     

CFD analysis of mixed convection porous square cavity

The analysis of conventional flow and transfer of heat in a square cavity which is porous in nature is being done by the numerical study and research. There is insulation at the vertical walls of the cavity and bottom surface wavy in nature upholds uniform temperature which is little more than that at the upper lid. The non-dimensional governing equations are solved using the finite volume method. Several inclination angles are examined from 0° to 180°. The isotherm and streamline distribution, including varieties of local and average Nusselt numbers with the angle of inclination, are exposed. The outputs of the observations indicate that average Nusselt number has direct relationship with a number of undulations and also inclination angle.     

Experimental investigation on heat transfer characteristics of heat exchanger with bubble fin assistance

ABSTRACT

Heat transfer area per unit volume (m²/m³) of heat exchangers decides the size of the heat exchangers, over the period of years heat exchangers have undergone numerous development in this aspect. One such attractive design is plate heat exchanger which is very compact and has high thermal effectiveness, whereas the flow nature of fluid through this type of heat exchangers is complex. The most common type of plate heat exchanger is chevron type which suffers from large pressure drop (Δp) at higher heat transfer rates, to overcome this problem bubble finned heat exchangers are designed. In this paper the performance of bubble finned heat exchanger is studied with single phase fluid flow condition. The comparative study of finned and no fin configuration shows that the former has 1.8 times higher rate of heat transfer at lower value of Reynold’s number, further the flow remains laminar hence the Δp is minimum.     

An empirical correlation for free convection in a window with a between-panes louvred blind

An empirical correlation for free convection in a double-glazed window with an enclosed Venetian blind is developed. The correlation makes use of an existing model from the literature (called the reduced slat length model) for the pure conduction asymptote. Published experimental data for night-time conditions (i.e. with no solar irradiation) are used to characterize the average Nusselt number at higher Rayleigh numbers. The asymptote blending method of Churchill and Usagi is used to combine these results into an empirical correlation that spans the full range of variables for which experimental data are currently available. The correlation is applicable to all common fill gases, and predicts the effects of Rayleigh number, slat angle and blind-to-enclosure width ratio on the average Nusselt number. The use of the correlation to predict the U-value of this complex fenestration is demonstrated. The results compare well with published measurements.     

A comparative numerical study on natural convection in inclined wavy and flat-plate solar collectors

The present study deals with the numerical analysis of natural convection heat transfer inside the inclined solar collectors. Two collectors are compared. In the first case, the collector has wavy absorber and in the second case, it has flat absorber. The solution was performed assuming the isothermal boundary conditions of absorbers and covers of collectors. CFDRC commercial software is used to simulate the laminar flow and thermal field. Governing parameters are taken as Rayleigh number (from 1×10⁶ to 5×10⁷), inclination angle (from 20° to 60°), wave length (from 1.33 to 4) and aspect ratio (from 0 to 4). Results are presented by streamlines, isotherms and local and mean Nusselt numbers. It is observed that flow and thermal fields are affected by the shape of enclosure and heat transfer rate increases in the case of wavy enclosure than that of flat enclosure.     

Modelling and thermal analysis of air-cooling system with fin pitch in IC engines

ABSTRACT

Excessive heat from the system in IC engines is removed by using the air-cooling system in order to avoid damaging and overheating of IC engines. Fins are most preferably used now to enhance convective heat transfer without the use of an excessive amount of the primary surface area. Choosing the configuration of the fin is very important in order to attain maximum effectiveness of the fin. The heat transfer from surfaces in general is enhanced by increasing the heat transfer area of the surface or the heat transfer coefficient between the surface and its surrounding. Extended fins are well known for enhancing the heat transfer in IC engines. Therefore, it is important for an air-cooled engine to utilise the fins more effectively to obtain uniform temperature distribution in the cylinder periphery. The aim of this paper is to investigate the thermal behaviour of the cooling system with fin pitch and surface geometry in ANSYS and modelling is done using solid works software, in order to compare its performance over the conventional fin design.     

Elastic stability of bi-axially loaded rectangular plates with a single circular hole

The finite-element method has been employed to determine the elastic buckling stresses of a bi-axially loaded perforated rectangular plate with dimensions a and b in the x- and y-directions, respectively. The considered perforation is a single circular hole whose center is located along the longitudinal axis of the plate. The considered plate has simply supported edges in the out-of-plane direction and is subjected to bi-axial uniformly distributed end loads (compressive load σ_x in the x-direction, and compressive or tensile load σ_y in the y-direction). Parameters considered in the study are the plate's aspect ratio a/b, the stress ratio ξ between the applied stresses in the y- and x-directions (ξ=σ_y/σ_x), the circular hole size d and location e_x.The study shows that, in most of the considered cases, the bigger the hole size d, the lesser the plate stability and the lesser the buckling stresses. It also shows that the plate aspect ratios, a/b, between 0.6 and 1.2 should be avoided for plates with large holes and negative ξ, due to the large reduction in the buckling stresses. The hole location should also be selected to be away from the loaded edge of the plate as much as possible (better to have e_x/b>1.0) to increase the buckling stresses and improve stability. The study demonstrates also that the increase in tension in the y-direction in bi-axially loaded plate with large hole (d/b>0.4) reduces its stability. This is in contrary to the expected increase in the stability due to the increase in tension which can be seen clearly in the cases of solid plates and plates with small holes.     

A study into optimization of stiffeners in plates subjected to shear loading

A great deal of attention has been focused on plates subjected to shear loading over the past decades. One main fact in design of such elements, which fall in the category of thin-walled structures, is their buckling behavior. Plate girders and recently shear walls are being widely used by structural engineers, as well as ship and aircraft designers. The role of stiffeners is proved to be vital in design of such structures to minimize their weight and cost.In this work, by using ANSYS finite element method of analysis, some 1200 plates are analyzed in order to study the role of stiffeners and to come up with some limits for an optimized design procedure. This eigenvalue method of analysis is first validated with the theoretical calculations and known cases for a wide range of typical panel geometries.The results show that the number of panels produced by intermediate transverse stiffeners should not be less than the value of plate's aspect ratio. In other words, the transverse stiffeners should divide the length of the plate to portions equal or less than its width.It is also shown that the optimum geometric properties of the stiffeners correspond to the point when the buckling shape of a plate changes from the overall mode to local mode. Furthermore, all stiffened plates, with a similar aspect ratio and number of stiffeners, have a specific value of EI_s/aD, for which the critical shear stress is optimal. In addition, some expressions to predict these properties are presented.     

Optimisation of electronic device coupled LiBr–H2O absorption heat pump system working at ambient temperature using Taguchi approach

The study attempts to optimise the parameters of a lithium bromide–water (LiBr–H₂O) miniature absorption refrigeration system using Taguchi approach for electronic cooling working at ambient conditions. Thermodynamic optimisation was performed to obtain the optimum coefficient of performance (COP) for heat removal of 100?W by using the Taguchi approach. Three factors were considered: generator temperature (T_g), condenser temperature (T_c) and absorber temperature (T_a), at three different levels. The result showed that the percentage contribution of generator temperature is more on COP. Optimisation of three significant heat exchangers: evaporator, condenser and absorber, was performed by using the Taguchi approach. The study was carried out individually for all the components by varying the hydraulic diameter and aspect ratio at three different levels. The results showed that the hydraulic diameter contributes more than the aspect ratio.     

Numerical study of heat transfer by laminar and turbulent natural convection in tall cavities of façade elements

Laminar and turbulent natural convection flow in a two-dimensional tall rectangular cavity heated from the vertical side has been investigated numerically for aspect ratios of 20, 40 and 80. The finite volume method was used to solve the conservation equations of mass, momentum and energy for Rayleigh numbers from 10² to 10⁸, the flow was considered either laminar or turbulent. For turbulent flow, four different turbulence models κ−ɛ were compared along with their experimental results for a cavity with an aspect ratio of 30, it was found that the better approach was with the one reported by Ince and Launder turbulent model [N. Ince, B. Launder, On the computation of buoyancy-driven turbulent flows in rectangular enclosures, Int. J. Heat Fluid Flow 10 (1989) 110–117]. The average Nusselt numbers as a function of Rayleigh numbers for the aspect ratios range of 20–80 were calculated and compared with five convective Nusselt number correlations reported from the literature. Convective Nusselt number correlations for laminar flow in the range of 10² ≤ Ra ≤ 10⁶ and for turbulent flow in the range of 10⁴ ≤ Ra ≤ 10⁸ were presented. This study will help to have more accurate heat transfer parameters for applications such as façade elements, insulating units, double-skin façades, etc.     

A CFD study of convection in a double glazed window with an enclosed pleated blind

A numerical study has been conducted of steady free convection in a double glazed window with a between-panes pleated cloth blind. The model geometry is based on a commercial product that is available on the consumer market in North America. The study considers the effects of Rayleigh number, enclosure aspect ratio, and blind geometry on the convective heat transfer. A simplified model of the coupled convective and radiative heat transfer is also presented. Sample results show that the average Nusselt number data from the CFD study can be combined with a one-dimensional model to closely predict the glazing-to-glazing U-value.     

Convection heat transfer in the cone calorimeter

Estimates of the convection heat transfer coefficient (HTC) in the cone calorimeter are obtained from a near-steady-state analysis of the temperature histories of horizontal coated steel plates. Corrections are applied to the derived HTC values to account for changes of the surface emissivity as a function of temperature. Furthermore, estimates are obtained of the additional heat losses through the sides of the sample holder and the insulation layer on which the test sample rests. The derived HTC values are analysed and correlations presented in the form of Nu=f(Ra) as well as h=f(T), where Nu is the plate Nusselt number, Ra is the plate Rayleigh number, h is the HTC and T is the plate temperature.     

Experimental investigation of free convection in roof solar collector

A test set-up was developed for determining the internal convective heat transfer coefficient and the induced air flowrate of a roof solar collector. The set-up was composed of an open-ended inclined rectangular channel. The tilt angle and air gap of channel were fixed at 30° and $\text{140}\text{mm}$, respectively. The inner channel width and length were 680 and $\text{1360}\text{mm}$, respectively. The upper plate was maintained at a uniform temperature and tests were carried out for a wide range of temperature varying between 40°C and 75°C. Data analysis were made to develop Nusselt and Reynolds number correlations as a function of Rayleigh number and aspect ratio (gap/length). The equations could be used for determining the mean convection heat transfer coefficient and the induced air flowrate of the roof solar collectors and a wide range of convective heat exchangers of similar geometry as well.     

Cardiff theory: Web panel aspect ratio limits and their relation with inclination angle of membrane tensile yield strength

Theoretical predictions of the ultimate shear resistance of slender-plate girders are broadly based on either Cardiff theory or Höglund’s theory. Cardiff theory assumes an equilibrium stress field (tension field) in the girder, which satisfies the theoretical conditions for a lower-bound strength prediction, provided the material possesses sufficient ductility for the stress field to be developed. Previous investigators did not determine the limits of web panel aspect ratio (b/d) or its relation to inclination angle of membrane tensile yielding strength (IAMTYS), which are important parameters in Cardiff theory. This paper presents a theoretical analysis to identify the limits of b/d that can be applied to Cardiff theory and their relation with c/b, the ratio of the distance where the plastic hinges form in the flanges (c) to the panel width (b). This analysis is based on previously published experimental results of ninety six steel plate girders subjected to shear. We make recommendations regarding (1) the optimum limits of panel aspect ratio for a given IAMTYS, and (2) the range of c/b values that can be used to predict the ultimate shear resistance using Cardiff theory.     

矩形翅片变形片翅片效率的数值解与性能分析

利用有限元方法对矩形翅片及三种几何变形片的翅片效率进行了数值求解, 求解结果经回归整理得出效率计算公式。数值计算结果表明,B2 形片虽然换热性能优于平片,但其翅片效率低于平片。C、D 形片翅片效率高于平片,且换热性能也优于平片。本文结果对高效换热翅片的优化设计研究有重要意义     

Role of soil inherent anisotropy in peak friction and maximum dilation angles of four sand-geosynthetic interfaces

Using a modified direct shear apparatus, an extensive experimental investigation is conducted into the influence of the inherent anisotropy of sand on the mobilization of the peak and critical state friction angles as well as the maximum dilation angle of the interfaces between an inherently anisotropic crushed sand and two woven geotextiles, one nonwoven geotextile, and one geomembrane. Experimental findings confirm that both peak and maximum dilation angles of sand-geosynthetic interfaces are affected from soil inherent anisotropy depending on the bedding plane inclination with respect to the shear plane. However, a unique critical state (residual) friction angle is attained for each interface type irrespective of the bedding plane inclination angle. Compiling results of a total of 141 tests, it is shown that a unique rule describes stress-dilation relationship of four different dense crushed sand-geosynthetic interfaces. The experimental data indicate that the ?_p vs. θ and ψ_max vs. θ curves are symmetrical with respect to θ?=?90° for the sand-woven geotextile and sand-geomembrane interfaces. Finally, it is shown that a constitutive equation by Pietruszczak and Mroz (2001) can predict the variation of ?_p with θ for the sand-woven geotextile and sand-geomembrane interfaces.     

Seismic uplift capacity of shallow horizontal strip anchor under oblique load using pseudo-dynamic approach

In this paper, an analytical method to compute the uplift capacity of an obliquely loaded horizontal strip anchor under both static and seismic conditions is described using the limit equilibrium method. The distribution of the soil reactions on a simple planar failure surface is obtained through the use of Kötter's equation, and the pseudo-dynamic approach is used to obtain the net seismic vertical uplift capacity factor for the unit weight component of the soil (F_γd). The results for the static and seismic vertical uplift capacity factors are determined for various combinations of input parameters, such as the load inclination, the soil friction angle, the embedment ratio, the soil amplification and both horizontal and vertical pseudo-dynamic seismic accelerations. It is observed that the orientation of the load significantly affects the seismic uplift capacity of the horizontal strip anchor. F_γd is seen to decrease with an increase in both horizontal and vertical seismic accelerations and soil amplification, whereas it is seen to increase with an increase in the embedment ratio and the soil friction angle, as expected. The results in terms of the non-dimensional net seismic uplift capacity factor are presented in graphical and tabular forms. The present results are compared and found to be in good agreement with similar results available in literature.