Dendritic fins optimization for a coaxial two-stream heat exchanger

This paper documents the fundamental relation between the maximization of global performance and the maleable (morphing) architecture of a flow system with global constraints. The example is the coaxial two-stream heat exchanger with flow through a porous bed in the annular space. It is shown that the constraints force the design toward heat exchangers with finite axial length, where additional improvements are derived from installing high-conductivity fins across the porous bed. The maximization of global performance is achieved through the optimization of the configuration of plate fins. Configurations with radial fins are optimized analytically and numerically. Configurations with branched fins are optimized numerically. It is shown that the best configuration (radial vs. branched) depends on the size of the heat exchanger cross-section. When the size is small, the best is the radial pattern. When the size exceeds a certain threshold, the best configuration is the optimized branched tree of fins.     

Optimum design-parameters of a heat exchanger having hexagonal fins

In the present study, the effects of the heights and widths of the hexagonal fins, streamwise and spanwise distances between fins, and flow velocity on the heat and pressure-drop characteristics were investigated using the Taguchi experimental-design method. Nusselt number and friction factor were considered as performance statistics. An L₁₈(2¹13⁷) orthogonal array was selected as the experimental plan for the five parameters mentioned above. While the optimum parameters were determined, the trade-off among goals was considered. First of all, each goal was optimized, separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be a fin width of 14 mm, a fin height of 150 mm, spanwise distance between fins of 20 mm, and streamwise distance between fins of 20 mm for a flow velocity of 4 m/s.     

Frost behavior on a fin considering the heat conduction of heat exchanger fins

A mathematical model is proposed for predicting frost behavior on a heat exchanger fin under frosting conditions, taking into account fin heat conduction. The change in the three-dimensional airside airflow caused by frost growth is reflected in this model. The numerical estimates of frost thickness are consistent with experimental data, with an error of less than 10%. Due to fin heat conduction, frost thickness decreases exponentially toward the fin tip, while considerable frost growth occurs near the fin base. When a constant fin surface temperature is assumed, the predicted frost thickness was larger by more than 200% at maximum, and the heat flux by more than 10% on average, compared to results obtained with fin heat conduction taken into account. Therefore, fin heat conduction could be an essential factor in accurately predicting frost behavior. To improve prediction accuracy under the assumption of constant fin surface temperature, the equivalent temperature (for predicting frost behavior) is defined to be the temperature at which the heat transfer rate neglecting fin heat conduction is the same as the heat transfer rate with fin heat conduction taken into consideration. Finally, a correlation for predicting the equivalent temperature is suggested.     

Parametric study on the performance of a heat exchanger with corrugated louvered fins

The Taguchi method is a well-known parametric study tool in engineering quality and experimental design. This study analyzes five experimental factors (flow depth, ratio of fin pitch and fin thickness, tube pitch, number of louvers and angle of louver) affecting the heat transfer and pressure drop of a heat exchanger with corrugated louvered fins using the Taguchi method. Fifteen samples are selected from experimental database and the heat transfer and flow friction characteristics are analyzed. The results show that flow depth, ratio of fin pitch and fin thickness and the number of the louvers are the main factors that influence significantly the thermal hydraulic performance of the heat exchanger with corrugated louvered fins. Therefore, these three factors are considered as the main factors for an optimum design of a heat exchanger.     

Investigation of a novel bayonet tube high temperature heat exchanger with inner and outer fins

In this paper, a novel bayonet tube high temperature heat exchanger (HTHE) with inner and outer fins is presented. It can be used in the ultra high temperature environment, such as hydrogen production, very high temperature reactor and externally fired combined cycle. Numerical investigation of heat transfer performance on the inside of bayonet element has been conducted for structure design. The numerical results suggest that the inner fin and inner tube should not be welded together. It is recommended that the air enters from the inner tube and exits from the annular space in the high temperature zone. A high temperature experimental system has been established to test the heat transfer and pressure drop characteristics of the HTHE. The surface area density of the tested HTHE is 6 times higher than that of the bare bayonet tube heat exchanger. The experimental results indicate that the mass flow rate on both sides and inlet temperature on the fuel gas side have a significant effect on the heat transfer rate and effectiveness, while the pressure drop ratios are mainly affected by the mass flow rate rather than the inlet temperature. Comparison between the tested HTHE and the similar HTHE without fins indicates that the proposed HTHE has a significant potential to improve the comprehensive heat transfer performance.     

Nusselt number correlations for a microchannel heat exchanger hot water supplier with S-shaped fins

Using 3D-CFD code, Nusselt number correlations for a microchannel heat exchanger (MCHE) with S-shaped fins used for hot water suppliers are obtained through numerical experiments and then validated. The supercritical carbon dioxide working fluid is assumed to operate around the pseudo-critical point, where fluid properties change radically. Calculations with 20 different temperatures are executed to produce Nusselt number correlations for each side. The fluid inlet temperature in each calculation is defined as 2 °C lower or higher than the constant wall temperature, respectively, for cold and hot side simulations. The small temperature difference of 2 °C is sufficiently small to regard thermal–hydraulic properties as constant. A new integrating method using the correlations to calculate the heat-transfer-performance is proposed. The resultant heat-transfer-performance is compared with that of another numerical result, which is reduced from large geometry and integration. The results agree within 3% error; the calculation accuracy of the method is confirmed. Experimental results with MCHE verify the correlations. The difference is approximately 5%. Using few computer resources, these Nusselt number correlations and the heat-transfer-performance calculation methods using correlation information are sufficiently accurate to evaluate heat exchangers.     

Conceptual optimization of a rotary heat exchanger with a porous core

The present article numerically optimizes the thermal performance of a rotary heat exchanger (RHEx) where its internal structure is modeled as a porous medium. The objective is to maximize the RHEx's heat transfer rate per unit of frontal surface area (q″). The flow velocity through the porous matrix respects Darcy's law. Two thermal conditions between the solid matrix and the fluid are considered: (i) local thermal equilibrium – LTE and (ii), non-local thermal equilibrium – NLTE. The numerical calculations, which are implemented using a finite volume formulation, allow us to optimize two design variables, the length L of the heat exchanger and the porosity φ. The numerical results show that the figure of merit is substantially affected by both design variables and that optimal values of L and φ can be obtained. The numerical experiments also show that the optimum porosity is not a function of the pressure difference driving the flow across the RHEx. The study ends by addressing the effects of the porosity distribution and differential periods between the hot and cold sides of RHEx on the figure of merit. The numerical results are supported by a scale analysis.     

Heat transfer enhancement of finned-tube heat exchanger using nozzle- and diffuser-shaped fins instead of straight fins

In this paper, a new method has been used to improve the heat transfer rate in the finned-tube heat exchanger with nozzle- and diffuser-shaped arrangement. For this study, the effect of several parameters was studied numerically. For the computational fluid dynamics simulation, the continuity, momentum, and energy equations were solved by the finite volume method using the standard k–ԑ model. The rate of heat transfer increases with the decreasing of fin bend radius (15 < R_fb < 20) for both nozzle-shaped fin and diffuser-shaped fin. By increasing of side temperature (600 < T_side < 900) and side Reynolds number (2000 < Re_side < 5000) the heat transfer rate increased for both nozzle- and diffuser-shaped fins. Results showed that a nozzle-shaped fin with a fin bend radius of 15 mm under the condition of Re_in = 20,000, T_side = 900 K, and Re_side = 3400 has a higher effect on heat transfer in comparison with the other types of fins. The maximum heat transfer rate was almost 39% and 35% for the nozzle-shaped fin with a bend radius of 15 mm and diffuser-shaped fin with a bend radius of 15 mm compared with the simple tube, respectively. Finally, correlational equations have been suggested to forecast the average Nu number as functions of various parameters of the tube equipped with different types of outer fins involving nozzle- and diffuser-shaped.     

Numerical investigation of heat transfer enhancement in a multitube thermal energy storage heat exchanger using fins

The application of a phase change material (PCM) as thermal energy storage observed unprecedented growth due to its large latent heat storage capacity at a constant temperature. However, the design of an energy storage heat exchanger is a challenging task because of the poor thermal conductivity of PCMs. In an effort to improve the heat exchanger design, this paper presents a numerical performance investigation of a PCM-based multitube heat exchanger incorporated with two new fin configurations. The analysis of the results shows that the placement of fins is one of the important aspects, which needs to be cogitated in the design of heat exchangers.     

Numerical investigation of fluid flow and heat transfer over louvered fins in compact heat exchanger

Numerical investigation of fluid flow and heat transfer characteristics over louvered fins and flat tube in compact heat exchangers is presented in this study. Three-dimensional simulations of single and double row tubes with louvered fins have been conducted. Simulations are performed for different geometries with varying louver pitch, louver angle, fin pitch and tube pitch and for different Reynolds number. Conjugate heat transfer and conduction through the fins are considered. The air-side performance of heat exchanger is evaluated by calculating Stanton number and friction factor. The results are compared with experiment and a good agreement is observed. The local Nusselt number variation along the top surface of the louver is calculated and effects of geometrical parameters on the average heat transfer coefficient is computed. Design curves are obtained which can used to predict the heat transfer and the pressure drop for a given louver geometry.     

A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins

An experimental energy storage system has been designed using a horizontal concentric tube heat exchanger incorporating a medium temperature phase change material (PCM) Erythritol, with a melting point of 117.7 °C. Three experimental configurations, a control system with no heat transfer enhancement and systems augmented with circular and longitudinal fins have been studied. The results presented compare the system heat transfer characteristics using isotherm plots and temperature-time curves. The system with longitudinal fins gave the best performance with increased thermal response during charging and reduced subcooling in the melt during discharging. The experimentally measured data for the control, circular finned and longitudinal finned systems have been shown to vindicate the assumption of axissymmetry (direction parallel to the heat transfer fluid flow) using temperature gradients in the axial, radial and angular directions in the double pipe PCM system.     

空气横掠矩形翅片椭圆管束换热规律的数值研究

采用Fluent软件对矩形翅片椭圆管束空气侧的对流换热情况进行了三维数值模拟,获得了不同流速下翅片表面温度分布,分析了迎面风速与换热系数之间的关系,随着速度的增大,空气侧的换热系数增加,并拟合了换热计算公式。同时分析了不同翅片间距对换热的影响因素,随着翅片间距的增大,空气侧换热系数增加,而且随着Rg数的增加,换热的强化更加明显。     

Performance analysis and optimization of straight taper fins with variable heat transfer coefficient

In the present paper, the thermal analysis and optimization of straight taper fins has been addressed. With the help of the Frobenius expanding series the temperature profiles of longitudinal fin, spine and annular fin have been determined analytically through a unified approach. Simplifying assumptions like length of arc idealization and insulated fin tip condition have been relaxed and a linear variation of the convective heat transfer coefficient along the fin surface has been taken into account. The thermal performance of all the three types of fin has been studied over a wide range of thermo-geometric parameters. It has been observed that the variable heat transfer coefficient has a strong influence over the fin efficiency. Finally, a generalized methodology has been pointed out for the optimum design of straight taper fins. A graphical representation of optimal fin parameters as a function of heat duty has also been provided.     

Analysis and optimization of convective trapezoidal profile pin fins with internal heat generation

An analysis of trapezoidal profile convective pin fins, with internal heat generation density is presented. The solution of the optimal problem is also given, when either the desired heat dissipation rate or the volume of the pin is specified. The results are presented graphically and in polynomial forms that are particularly useful for computerized calculations. The effect of the fin's profile and thermal conductivity upon the optimum dimensions is discussed. An example serves to demonstrate the usefulness of the method.     

Thermal performance analysis and optimum design parameters of heat exchanger having perforated pin fins

This paper reports the heat transfer enhancement and corresponding pressure drop over a flat surface equipped with circular cross section perforated pin fins in a rectangular channel. The channel had a cross section area of 100–250 mm². The experiments covered the following ranges: Reynolds number 13500–42,000, clearance ratio (C/H) 0, 0.33 and 1 and interfin spacing ratio (S_y/D) 1.208, 1.524, 1.944 and 3.417. Correlation equations were developed for the heat transfer, friction factor and enhancement efficiency. The experimental results showed that the use of circular cross section pin fins may lead to heat transfer enhancement. Enhancement efficiencies varied between 1.4 and 2.6 depending on clearance ratio and interfin spacing ratio. Using a Taguchi experimental design method, optimum design parameters and their levels were investigated. Nusselt number and friction factor were considered as performance parameters. An L₉(3³) orthogonal array was selected as an experimental plan. First of all, each goal was optimized separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be Reynolds number of 42,000, fin height of 50 mm and streamwise distance between fins of 51 mm.     

Multi-objective shape optimization of a heat exchanger using parallel genetic algorithms

We perform in this paper a multi-objective design optimization concerning the blade shape of a heat exchanger, considering the coupled solution of the flow/heat transfer processes. For this, a genetic algorithm is used. The aim of the procedure is to find the geometry most favorable to simultaneously maximize heat exchange while obtaining a minimum pressure loss. An in-house computer package, called OPAL, performs the optimization process in a fully automatic manner. It calls the pre-processor to generate the computational geometry as well as the mesh, it then performs the numerical simulation of the coupled fluid flow/heat transfer problem using Fluent, calculates the output parameters, and iterates the procedure. The genetic algorithm relies on a relatively large number of simulations, which may result in a considerable computational effort, depending on the configuration. The procedure can thus be performed in parallel on a Linux PC cluster to reduce user waiting time. A nearly optimal speed-up is obtained for the present configuration.     

Thermodynamic optimization of a coiled tube heat exchanger under constant wall heat flux condition

In this paper the second law analysis of thermodynamic irreversibilities in a coiled tube heat exchanger has been carried out for both laminar and turbulent flow conditions. The expression for the scaled non-dimensional entropy generation rate for such a system is derived in terms of four dimensionless parameters: Prandtl number, heat exchanger duty parameter, Dean number and coil to tube diameter ratio. It has been observed that for a particular value of Prandtl number, Dean number and duty parameter, there exists an optimum diameter ratio where the entropy generation rate is minimum. It is also found that with increase in Dean number or Reynolds number, the optimum value of the diameter ratio decreases for a particular value of Prandtl number and heat exchanger duty parameter.     

Numerical investigation on the multi-objective optimization of a shell-and-tube heat exchanger with helical baffles

An improved method combining numerical simulation with multi-objective genetic algorithm (MOGA) was applied to study the flow and heat transfer characteristics of shell-and-tube heat exchanger with helical baffles (STHXsHB). It overcomes the dependence on empirical correlations. The helix angle and overlapped degree of helical baffles were chosen as optimization parameters, while the overall heat transfer coefficient K and pressure drop ΔP of STHXsHB were optimized by MOGA. The results showed that both overall heat transfer coefficient K and pressure drop ΔP varied adversely with the helix angles. The pressure drop ΔP was favorably affected by the overlapped degrees. The overall heat transfer coefficient K did not vary significantly with the overlapped degree. Three optimum configurations were obtained by the MOGA to maximize the overall heat transfer coefficient K and minimize the shell-side pressure drop ΔP. Compared with the original heat exchanger, the overall heat transfer coefficient K increased averagely by 28.3%, while the average pressure drop reduced averagely by 19.37%.     

Thermodynamic optimization of cross flow plate-fin heat exchanger using a particle swarm optimization algorithm

This study explores the use of particle swarm optimization (PSO) algorithm for thermodynamic optimization of a cross flow plate-fin heat exchanger. Minimization of total number of entropy generation units for specific heat duty requirement under given space restrictions, minimization of total volume, and minimization of total annual cost are considered as objective functions and are treated individually. Based on the applications, heat exchanger length, fin frequency, numbers of fin layers, lance length of fin, fin height and fin thickness or different flow length of the heat exchanger are considered for optimization. Heat duty requirement constraint is included in the procedure. Two application examples are also presented to demonstrate the effectiveness and accuracy of the proposed algorithm. The results of optimization using PSO are validated by comparing with those obtained by using genetic algorithm (GA). Parametric analysis is also carried out to demonstrate the effect of heat exchanger dimensions on the optimum solution. The effect of variation of PSO parameters on convergence and optimum value of the objective has also been presented.     

Fin spacing optimization of a fin-tube heat exchanger under frosting conditions

Optimal values of the design parameters for a fin-tube heat exchanger of a household refrigerator under frosting conditions are proposed to improve its thermal performance and extend its operating time. In the optimization procedure, fin spacings of the heat exchanger are selected as the design parameters, and the average heat transfer rate, frost mass, and operating time are considered to be objective functions. The response surface and Taguchi methods are employed to optimize the design parameters. As a result, the average heat transfer rate and operating time of the optimum models increases by up to 6.3% and 12.9% compared to that of the reference model, respectively.