Modeling for predicting frosting behavior of a fin–tube heat exchanger

A mathematical model is proposed to evaluate the frosting behavior of a fin–tube heat exchanger under frosting conditions. Empirical correlations of the heat transfer coefficients for the plate and tube surfaces and a diffusion equation for the frost layer are used to establish the model. The correlations for the heat transfer coefficients, derived from various experimental data, were obtained as functions of the Reynolds number and Prandtl number. The proposed model is validated by comparing the numerical results with experimental data for the frost thickness, frost accumulation, and heat transfer rate. The numerical results agree well with the experimental data. It is also found that this model can be applied to evaluate the thermal performance of a common fin–tube heat exchanger under frosting conditions.     

Air side heat transfer coefficients of discrete plate finned-tube heat exchangers with large fin pitch

The objective of this study is to investigate the heat transfer characteristics of discrete plate finned-tube heat exchangers with large fin pitches. Thirty-four heat exchangers were tested with variations of fin pitches, the number of tube rows, fin alignment, and vertical fin space. The j-factor of the discrete plate finned-tube exchanger was analyzed as a function of coil geometry and then compared with that of the continuous plate finned-tube heat exchanger. For fin pitches of 7.5–15 mm, the j-factors of the discrete plate finned-tube heat exchangers were 6.0–11.6% higher than those of the continuous plate finned-tube heat exchangers. Two separate correlations for the j-factor were developed for the inline and the staggered fin alignment in the discrete plate finned-tube heat exchangers to predict the measured data within a relative deviation of 2.9%.     

Numerical prediction of flow and heat transfer in a channel in the presence of a built-in circular tube with and without an integral wake splitter

A numerical investigation was carried out to study the heat transfer behavior of a circular tube in cross-flow configuration with a longitudinal fin attached at the rear of the tube. The investigated configuration is intended to model either an element of a cross-flow heat exchanger or an element of the array of pin fins. The longitudinal finning of a circular tube is assumed to be in a configuration where the fin is attached at the back of the circular tube. The longitudinal fins, built-in with the tubes, are called integral splitter plates. The splitter plate creates a streamlined extension of the circular tube. It brings about enhancement of heat transfer from the tube surface. A reduction in the size of the wake zone in comparison with the wake of a circular tube is observed. Narrowing of the wake zone reduced convective heat transfer from the tube surface but the splitter plate itself generated an extra fin area for conduction. Overall, there is an improvement in heat transfer past the circular tube with an integral splitter plate compared with the case of flow past a circular tube without a splitter plate. Flow and heat transfer results are presented for three different chord lengths of the splitter plate and three different values of the Reynolds numbers (500, 1000 and 1500). The heat transfer enhancement obtained by finning was compared with that obtained by increasing the diameter of the unfinned tubes.     

单向开缝翅片管换热器传热与阻力性能的数值模拟及试验研究

对一种单向开缝翅片管换热器进行了数值模拟及试验研究,分析了不同翅片间距及管径下单向开缝翅片管换热器的传热与阻力性能的变化规律。数值模拟和试验结果的对比表明,采用数值模拟方法研究单向开缝翅片管换热器的传热与阻力性能是可行的。     

Numerical investigation of thermal enhancement of plate fin type heat exchanger with creases and holes in construction machinery

The present study aims to develop a new shape of plate fin heat exchanger by applying ceases and holes on the plate fin which is used in the construction machinery under the poor environment where the dust content is high and much extraneous materials are generated. In the present study, the louver fin heat exchanger, the plate fin heat exchanger and the heat exchanger of the shape newly proposed in this study were analyzed comparatively using three-dimensional numerical simulation. The fully developed airflow in the unit cell of heat exchangers considered was modeled with k–ω turbulence model using steady incompressible Reynolds-Averaged Navier–Stokes (RANS) equations. The calculated pressure drop and heat transfer capacity for the louver fin heat exchanger, the plate fin heat exchanger and the heat exchanger of the shape newly proposed in this study were assessed in terms of the Fanning friction factor (f factor) and Colburn j-factor (j factor) while the overall performance was estimated using the volume and area goodness factors.     

A three-dimensional numerical study and comparison between the air side model and the air/water side model of a plain fin-and-tube heat exchanger

CFD is becoming an important heat exchanger research technique. It constitutes an inexpensive prediction method, avoiding the need of testing numerous prototypes. Current work in this field is mostly based on air flow models assuming constant temperature of fin-and-tube surface. The purpose of this paper is to present an enhanced model, whose innovation lies in considering additionally the water flow in the tubes and the conduction heat transfer through the fin and tubes, to demonstrate that the neglect of these two phenomena causes a simulation result accuracy reduction.3-D Numerical simulations were accomplished to compare both an air side and an air/water side model. The influence of Reynolds number, fin pitch, tube diameter, fin length and fin thickness was studied. The exchanger performance was evaluated through two non-dimensional parameters: the air side Nusselt number and a friction factor. It was found that the influence of the five parameters over the mechanical and thermal efficiencies can be well reported using these non-dimensional coefficients. The results from the improved model showed more real temperature contours, with regard to those of the simplified model. Therefore, a higher accuracy of the heat transfer was achieved, yielding better predictions on the exchanger performance.     

Effects of winglets to augment tube wall heat transfer in louvered fin heat exchangers

The louvered fin heat exchanger, a type of compact heat exchanger, has been used heavily in the automotive and air conditioning industries for the last several decades. The majority of past research, aimed towards improving louvered fin exchanger efficiency, has focused on optimizing various parameters of the louvered fin. The experimental study presented in this paper concentrates instead on augmenting the heat transfer along the tube wall of the compact heat exchanger through the use of winglets placed on the louvers. The experiments were completed on a 20 times scaled model of an idealized louvered fin exchanger with a fin pitch to louver pitch ratio of 0.76 and a louver angle of 27°. The Reynolds numbers tested, based on louver pitch, were between 230 and 1016. A number of geometrical winglet parameters, including angle of attack, aspect ratio, direction, and shape, were all evaluated based on heat transfer augmentation, friction factor augmentation, and efficiency index (combination of both augmentations). In an attempt to optimize these winglet parameters, tube wall heat transfer augmentations as high as 39% were achieved with associated friction factor augmentations as high as 23%.     

Determination of heat transfer coefficients for cylindrical products exposed to forced-air cooling

An analytical model for estimation of transient heat transfer coefficients in forced-air precooling experiments of cylindrically shaped grapes, using a lumped capacitance approach were addressed and investigated. In order to determine transient heat transfer coefficients, the centre transient temperature measurements during forced-air precooling were used. Experiments involved cooling individual grapes in air flow without water losses. The individual grapes were instrumented with several interior thermocouples for measuring the centre transient temperature response during cooling. The transient values of the heat transfer coefficient history for five different air velocities were found to be about 21–40 W/m² K. These values were in good agreement with the values predicted using well-known Nusselt-Reynolds empirical correlation for forced convection. The present technique has the capability of determining transient heat transfer coefficients in a single transient experiment.     

An experimental study in determining the local heat transfer coefficients for the plate finned-tube heat exchangers

A three-dimensional inverse problem in determining the local heat transfer coefficients for the plate finned-tube heat exchangers utilizing the steepest descent method (SDM) and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the measured temperature distributions on fin surface by infrared thermography.Two different tube arrangements (i.e. in-line and staggered) with different fin pitch and air velocity are considered and the corresponding local heat transfer coefficients are to be determined. Results show that some interesting phenomena of the local heat transfer coefficients for the finned surface are found in the work and the averaged heat transfer coefficient of the staggered configuration is about 8–13% higher than that of the in-line configuration.     

Numerical study of the fin efficiency and a modified fin efficiency formula for flat tube bank fin heat exchanger

In this paper, a three-dimensional numerical heat transfer analysis has been performed in order to obtain the temperature distribution and the fin efficiency using the experimentally determined local heat transfer coefficients from the naphthalene sublimation technique and heat and mass transfer analogy. The influences of the fin material, fin thickness, and transversal tube pitch on the fin efficiency are studied for flat tube bank fin heat exchangers. The fin efficiency, obtained by a numerical method using the averaged heat transfer coefficient, is compared with that using the local heat transfer coefficient. The reliability of the generally used formula for fin efficiency is tested also, and then a modified fin efficiency formula with a new equivalent fin height is provided. The results show that the difference between the fin efficiency obtained by the numerical method using the local heat transfer coefficient and the fin efficiency using the averaged heat transfer coefficient is small, but the fin efficiency obtained by the generally used formula is lower than that obtained by the numerical method using the local heat transfer coefficient; the fin efficiency obtained by the modified formula matches very well with the fin efficiency obtained by the numerical method using the local heat transfer coefficient. The modified formula for the fin efficiency calculation is more reliable, and it can be applied directly to the design of a flat tube bank fin heat exchanger and also will be useful in engineering applications.     

Parametric and design optimization investigation of a wavy fin and tube air heat exchanger using the T-G technique

The focus of this paper is to optimize the air-side performance of a wavy fin and tube heat exchanger at different design parameters on an individual target response using the Taguchi method. However, a statistical concept, gray relational analysis, is also studied for combined optimization, considering all target responses at a time. Based on the heat exchanger requirement, parametric study for the air-side is regarded as a more significant heat transfer and lower frictional factor. Experimental correlations were available and used for the 27 orthogonal runs. Investigation revealed the highest 47.06% fin pitch, 37.24% fin pitch, 25.46% air velocity, and 23.9% fin thickness contribution ratio for the target response of friction factor (TPF), heat transfer coefficient, and Colburn factor, respectively, with the application of the Taguchi method in a heat exchanger. GRG gives an optimum set of design parameters, A3B3C2D1E3F2G1, for wavy fin and tube of fin pitch of 6 mm, tube row number of 6, waffle height 1.8 mm, fin thickness 0.12 mm, and air velocity 5 m/s. Also, longitudinal tube pitch is 27.5 mm, and transverse tube pitch of 24.8 mm, at which TPF is maximum while the friction factor is minimal. The Colburn factor is the most significant, minor friction factor, and the heat transfer coefficient and TPF are the most considerable in GRG. Hence, an improved heat transfer performance design of a wavy fin and tube heat exchanger is achieved using the above techniques.     

A method to reduce the flow depth of a plate heat exchanger without a loss of heat transfer performance

With the aim of improving heat exchanger compactness, this study investigates how the optimum configuration of an air–liquid plate heat exchanger changes as the heat exchanger depth decreases. In this respect, optimization of an air–liquid plate heat exchanger with a given frontal area and a given depth is achieved. The optimum fin pitch and plate pitch are obtained to maximize the heat transfer rate based on heat transfer and pressure loss correlations in finned channels. Then, the focus of this study is placed on how the optimum channel configuration changes when the heat exchanger depth decreases for compactness. The results illustrate that the heat transfer performance can remain unchanged if the geometric parameters, such as the plate thickness, the plate pitch, the fin thickness, and the fin pitch, are reduced proportionally to the square root of the flow depth reduction given that the flow remains laminar. This finding is arranged into a simple scaling rule to obtain the configuration of a more compact heat exchanger from an existing configuration. In addition, the scaling arguments are extended to practical situations where the fin thickness and the plate thickness are not properly reduced following the scaling rule due to limitations on available material thicknesses.     

A three-dimensional inverse problem in imaging the local heat transfer coefficients for plate finned-tube heat exchangers

A three-dimensional inverse heat conduction problem in imaging the local heat transfer coefficients for plate finned-tube heat exchangers utilizing the steepest descent method and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the simulated measured temperature distributions on fin surface by infrared thermography.It is assumed that no prior information is available on the functional form of the unknown local heat transfer coefficients in the present study. Thus, it can be classified as function estimation for the inverse calculations.Two different heat transfer coefficients for in-line tube arrangements with different measurement errors are to be estimated. Results show that the present algorithm can obtain the reliable estimated heat transfer coefficients.     

Experimental study on heat and mass transfer characteristics of louvered fin-tube heat exchangers under wet condition

An experimental study was conducted to investigate the effect of a tube row, a fin pitch and an inlet humidity on air-side heat and mass transfer performance of louvered fin-tube heat exchangers under wet condition. Experimental conditions were varied by three fin pitches, two rows, and two inlet relative humidities. From the experimental results, it was found that the heat transfer performance decreased and the friction increased with the decrease of a fin pitch, for 2 row heat exchanger. The effect of a fin pitch on heat transfer performance was negligible with 3 row heat exchanger. The change in a relative humidity was not affected heat transfer and friction. However, the mass transfer performance was slightly decreased with the increase of a relative humidity and with the decrease of a fin pitch. The mass transfer performance decreased with the decrease of a fin pitch. The mass transfer performance of the louvered fin-and-tube heat exchanger was different according to the number of a tube row.     

Numerical computation for parallel plate thermoacoustic heat exchangers in standing wave oscillatory flow

A simplified computational method for studying the heat transfer characteristics of parallel plate thermoacoustic heat exchangers is presented. The model integrates the thermoacoustic equations of the standard linear theory into an energy balance-based numerical calculus scheme. Details of the time-averaged temperature and heat flux density distributions within a representative domain of the heat exchangers and adjoining stack are given. The effect of operation conditions and geometrical parameters on the heat exchanger performance is investigated and main conclusions relevant for HX design are drawn as far as fin length, fin spacing, blockage ratio, gas and secondary fluid-side heat transfer coefficients are concerned. Most relevant is that the fin length and spacing affect in conjunction the heat exchanger behavior and have to be simultaneously optimized to minimize thermal losses localized at the HX-stack junctions. Model predictions fit experimental data found in literature within 36% and 49% respectively at moderate and high acoustic Reynolds numbers.     

椭圆管散热器传热及阻力性能试验研究

对空气横掠片距不相等的叉排椭圆翅片管散热器的传热及阻力性能进行了试验研究,得到试件在一系列工况下的传热与管外流动阻力数据,并对试验数据进行分析计算,从总传热系数K中分离出管外空气侧的对流换热系数h,给出有工程应用价值的管外换热准则关系式及管外阻力准则关系式。认为椭圆管管外的平均换热效果优于圆管。在相同的流通截面积下椭圆管传热周边较长,换热面积相应增加,因此结构上允许布置得更紧凑。     

Prediction of heat transfer coefficient on the fin inside one-tube plate finned-tube heat exchangers

The finite difference method in conjunction with the least-squares scheme and the experimental temperature data is proposed to predict the average heat transfer coefficient and the fin efficiency on the fin inside one-tube plate finned-tube heat exchangers for various air speeds and the temperature difference between the ambient temperature and the tube temperature. Previous works showed that the heat transfer coefficient on this rectangular fin is very non-uniform. Thus the whole plate fin is divided into several sub-fin regions in order to predict the average heat transfer coefficient and the fin efficiency on the fin from the knowledge of the fin temperature recordings at several selected measurement locations. The results show that the surface heat flux and the heat transfer coefficient on the upstream region of the fin can be markedly higher than those on the downstream region. The fin temperature distributions depart from the ideal isothermal situation and the fin temperature decreases more rapidly away from the circular center, when the frontal air speed increases. The average heat transfer coefficient on the fin increases with the air speed and the temperature difference between the ambient temperature and the tube temperature. This implies that the effect of the temperature difference between the tube temperature and the ambient temperature is not negligent.     

Numerical study on air-side performance of an integrated fin and micro-channel heat exchanger

A new type of aluminum heat exchanger with integrated fin and micro-channel has been proposed. The air-side heat transfer and flow characteristics of the integrated fin and micro-channel heat exchanger are systematically analyzed by a 3D numerical simulation. The effect of flow depth, fin height, fin pitch and fin thickness at different Reynolds number is evaluated by calculating Colburn factor j and Fanning friction factor f. A parametric study method is used to analyze the fin designed parameters affecting the performance of the heat exchanger. The results show that the contribution ratio of the fin geometries in descending order is flow depth, fin pitch, fin height and fin thickness. The air-side performance of the integrated fin and micro-channel heat exchanger is compared with that of the multi-louver fin micro-channel heat exchanger and the wavy fin micro-channel heat exchanger.     

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.     

Three-dimensional numerical study of wavy fin-and-tube heat exchangers and field synergy principle analysis

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.