Flow visualization of annular and delta winlet vortex generators in fin-and-tube heat exchanger application

This study presents flow visualization and frictional results of enlarged fin-and-tube heat exchangers with and without the presence of vortex generators. Two types of vortex generators and a plain fin geometry were examined in this study. For plain fin geometry at Re=500, the horseshoe vortex generated by the tube row is not so pronounced, and the horseshoe vortex separates into two streams as it flows across the second row and consequently loses its vortical strength. This phenomenon may supports the “maximum phenomenon” in low Reynolds number region reported by previous studies. With the presence of annular vortex generator, the presence of a pair of longitudinal vortices formed behind the tube is seen. The strength of the counter-rotating vortices increases with the annular height and the strength of the longitudinal vortices is so strong that may swirl with the horseshoe vortices and other flow stream. For the same winlet height, the delta winlet shows more intensely vortical motion and flow unsteadiness than those of annular winlet. This eventually leads to a better mixing phenomenon. However, it is interesting to know that the corresponding pressure drops of the delta winlet are lower than those of annular winlet. Compared to the plain fin geometry, the penalty of additional pressure drops of the proposed vortex generators is relatively insensitive to change of Reynolds number.     

Second law analysis of periodic heat conduction through a wall

Periodic heat conduction through a wall is a simple model for the behaviour of a building wall submitted to climatic temperature changes. Beyond the well-known definition of heat transfer, the present concern is thermodynamics, through two quantities. The first quantity is the total entropy generation (total over time period and wall thickness). Formal derivation shows that from this point of view also, the phenomenon is the superposition of stationary linear heat-conduction plus periodic heat-diffusion around a uniform temperature. The second quantity is defined by considering ideal Carnot cycles that would maintain the inner space at a prescribed temperature. Parameters that influence either quantities are explored, evidencing values of the wall thickness beyond which there does not seem to be any interest in increasing the wall thickness.     

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 of fouling on cross-flow heat exchanger tubes with conjugated heat transfer approach

Although fouling on heat exchanger tubes is extensively investigated, due to the lack of energy resources, the effects of fouling on heat exchangers is still an important area of study and gaining more and more attention every day. In this study we investigated the effects of fouling on heat transfer and flow structures numerically for cross-flow heat exchanger tube geometry. The distributions of temperature, heat transfer coefficient and heat flux at the surface of fouling were obtained for single and double layer fouling cases. In the analysis, Reynolds number and the blockage ratio were fixed to 100 and 0.1 respectively. We used ANSYS software in our analyses and compared some of our results with the literature.     

Novel structural designs of fin-tube heat exchanger for PEMFC systems based on wavy-louvered fin and vortex generator by a 3D model in OpenFOAM

Heat management is crucial to the stable and high-efficiency operation of proton exchange membrane fuel cell (PEMFC) system. However, fin-tube heat exchangers (FTHE) of traditional internal combustion engine vehicles require further optimizations to be applicable to PEMFC vehicles. In the paper, a three-dimensional steady-state radiator model is developed in OpenFOAM to investigate three novel structural designs based on wavy-louvred (WL) fin and vortex generators (VGs). The established model has been carefully validated against experimental data and correlation reference. To comprehensively evaluate radiator performances, the air side heat transfer coefficient, pressure drop, outlet air temperature, heat flux, and JF factor are adopted. It is found that the FTHE with L-VGs has the highest heat transfer coefficient while the FTHE with WL-VGs has the highest pressure drop. The temperature, velocity, and pressure distribution are further demonstrated to reveal performance enhancement mechanisms. It is seen that the heat exchangers with additional VGs produce two sections of high-temperature wakes near the wall, which not only promotes the heat convection but also contributes to the heat exchange in the nearby area. Meanwhile, a low-speed vortex zone behind VGs appears and generates longitude vortex, making the air stream stay longer at fin surfaces. The air flow in FTHE with WL is not as much separated as the conventional FTHE since the zigzag wavy louver restricts flow separation. The paper gives valuable suggestions for cooling capability improvement and radiator volume diminution.     

加涡产生器的管片式换热器换热与阻力特性的数值研究

通过数值模拟的方法,研究了小翼式涡产生器对错排圆管管片式换热芯子换热与阻力特性的影响,比较了光板与加涡产生器强化板芯的速度场、横向平均Nu数以及平均对流换热系数、阻力系数的变化规律,为进一步提高其换热性能、改进翅片结构、设计新型换热器提供了理论依据。     

Thermohydraulic performance of a new internal twisted ribs automobile exhaust heat exchanger for waste heat recovery applications

The performance of the thermoelectric-based waste heat recovery (WHR) system in an automobile greatly depends on the amount of heat extracted by the exhaust heat exchanger (EHE). In the present study, the thermohydraulic performance of the EHE having twisted ribs and the pressure drop across the entire heat exchanger have been experimentally investigated. The experiments were repeated for the various geometric parameters such as twist ratio (4-8), angle of attack (30°-90°), and pitch ratio (6-10) on the Reynolds number within the range of 2300 to 25,000. The heat transport and fluid flow characteristics were compared with an internally smooth EHE using the thermohydraulic performance parameter. The maximum heat transfer rate was improved at 164.22%. However, the specification of the twisted rib for superior performance has been obtained at twist ratio of 4 and pitch ratio of 8 with 60° angle of attack. The highest thermohydraulic performance parameter value revealed as 1.93 at the same configuration. With the change in twist ratio, the pitch ratio, angle of attack, and the heat transfer rate increased by 39.52%, 60.85%, and 40.70%, respectively. Thus, the efficient heat transfer with the twisted rib would improve the extent of WHR in automobiles.     

The heat transfer and pressure loss characteristics of a heat exchanger for recovering latent heat (the heat transfer and pressure loss characteristics of the heat exchanger with wing fin)

In recent years the requirement for reduction of energy consumption has been increasing to solve the problems of global warming and the shortage of petroleum resources. A latent heat recovery type heat exchanger is one of the effective methods of improving thermal efficiency by recovering latent heat. This paper described the heat transfer and pressure loss characteristics of a latent heat recovery type heat exchanger having a wing fin (fin pitch: 4 mm, fin length: 65 mm). These were clarified by measuring the exchange heat quantity, the pressure loss of heat exchanger, and the heat transfer coefficient between outer fin surface and gas. The effects of condensate behavior in the fins on heat transfer and pressure loss characteristics were clarified. Furthermore, the equations for predicting the heat transfer coefficient and pressure loss which are necessary in the design of the heat exchanger were proposed. ©2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(4): 215–229, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20154     

蓄热式管壳换热器传热特性研究

针对热电联产机组供热期发电负荷受供热量限制,机组调峰能力下降、电力系统弃风弃光现象严重的问题,设计了一种新型蓄热式管壳换热器.利用相变材料蓄/放热过程中温度接近恒定、释放潜热量大等优点,选取石蜡为相交材料,换热器相交区作为换热单元,采用控制变量法,针对传热流体流速、相变材料导热系数及相变层厚度等关键因素,对换热单元的蓄...     

Heat transfer enhancement by using a rectangular wing vortex generator on the triangular shaped fins of a plate‐fin heat exchanger

The present numerical analysis pertains to the heat transfer enhancement in a plate‐fin heat exchanger employing triangular shaped fins with a rectangular wing vortex generator on its slant surfaces. The study has been carried out for three different angles of attack of the wing, i.e., 15°, 20° and 26°. The aspect ratio of the wing is not varied with its angle of attack. The flow considered herein is laminar, incompressible, and viscous with the Reynolds number not exceeding 200. The pressure and the velocity components are obtained by solving the continuity and the Navier– Stokes equations by the Marker and Cell method. The present analysis reveals that the use of a rectangular wing vortex generator at an attack angle of 26° results in about a 35% increase in the combined spanwise average Nusselt number as compared to the plate‐triangular fin heat exchanger without any vortex generator. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20285     

Transient heat transfer in a U-tube borehole heat exchanger

A transient heat transfer model has been development for a thermal response test (TRT) on a vertical borehole with a U-tube. Vertical borehole heat exchangers are frequently coupled to ground source heat pumps, which heat and cool buildings. The model provides an analytical solution for the vertical temperature profiles of the circulating fluid through the U-tube, and the temperature distribution in the ground. The model is verified with data sets from a laboratory sandbox and field TRTs, as well as a previously reported numerical solution. Unlike previous analytical models, the vertical profiles for the circulating fluid are generated by the model without any assumption of their functional form.     

Second law analysis of crossflow heat exchanger in the presence of axial dispersion in one fluid

In the present paper second law analysis of crossflow heat exchangers has been carried out in the presence of non-uniformity of flow. This non-uniformity is modeled with the help of axial dispersion model and takes into account the back mixing and flow maldistribution. An analytical model for exergy destruction has been evaluated for the cross-flow configuration. A wide range of study of the operating parameters and non-uniform flow on exergetic behavior of crossflow heat exchangers has been carried out. The results clearly bring out not only the reason behind the maximum entropy paradox in heat exchangers but also the proper perspective of exergy destruction and the consequent optimization of crossflow heat exchangers from the second law viewpoint.     

Effect of maximum and minimum heat capacity rate on entropy generation in a heat exchanger

This paper presents a theoretical analysis of a heat exchanger with a negligible fluid flow pressure drop to determine whether it is better to operate the heat exchanger with the minimum or maximum heat capacity rate of the hot fluid from entropy generation point of view. Entropy generation numbers are derived for both cases, and the results show that they are identical, when the heat exchanger is running at a heat capacity ratio of 0.5 with heat exchanger effectiveness equaling 1. An entropy generation number ratio is defined for the first time, which has a maximum value at ε = 1/(1+R) for any inlet temperature ratio. When R equals 0.1, 0.5 and 0.9, the entropy generation number ratio receives a maximum value at an effectiveness equaling 0.91, 0.67 and 0.526, respectively. When R=0.9, the entropy generation number ratio is the same for all inlet temperature ratios at ε=0.8. The results show that the entropy generation number ratio is far from 1 depending on the inlet temperature ratio of the cold and hot fluid. The results are valid for parallel‐flow and counterflow heat exchangers. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical studies on flow and heat transfer in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger

The flow and heat transfer characteristics of synthesis gas (syngas) in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger under different operating pressures, inlet velocities and pitches are investigated numerically. The three-dimensional governing equations for mass, momentum and heat transfer are solved using a control volume finite difference method. The realizable k-ε model is adopted to simulate the turbulent flow and heat transfer in heat exchangers. There flows syngas in the channels consisting of the membrane helical coils or membrane serpentine tubes, where the operating pressure varies from 0.5 to 3.0 MPa. The numerically obtained heat transfer coefficients for heat exchangers are in good agreement with experimental values. The results show that the syngas tangential flow in the channel consisting of membrane helical coils is significant to the heat transfer enhancement to lead to the higher average heat transfer coefficient of membrane helical-coil heat exchanger compared to membrane serpentine-tube heat exchanger. The syngas tangential velocity in the membrane helical-coil heat exchanger increases along the axial direction, and it is independent of the gas pressure, increasing with the axial velocity and axial pitch rise and decreasing with the radial pitch rise.     

Second law analysis in a three dimensional lid-driven cavity

Three dimensional analyses of laminar mixed convection and entropy generation in a cubic lid-driven cavity have been performed numerically. Left side of cavity moves in + y (Case I) or −y (Case II) direction. The cavity is heated from left side and cooled from right while other surfaces are adiabatic. Richardson number is the main parameter which changes from 0.01 to 100. Prandtl number is fixed at Pr = 0.71. Results are presented by isotherms, local and mean Nusselt number, entropy generation due to heat transfer and fluid friction, velocity vectors and Bejan number. Total entropy generation contours are also presented. It is found that direction of lid is an effective parameter on both entropy generation and heat and fluid flow for low values of Richardson number but it becomes insignificant at high Richardson number.     

Second law optimization of convective heat transfer through a duct with constant heat flux

A second law analysis is carried out on convective heat transfer from a fluid flowing in a duct with constant heat flux. The entropy generated is expressed as a function of the initial temperature difference and the frictional pressure drop. Since the loss in available energy is directly proportional to the entropy generated, an optimum value of the initial temperature difference is found where the entropy generated is the minimum. A similar optimum is found for the ratio of heat transfer to pumping power. An optimum fluid velocity which corresponds to the minimum loss of available power is recommended.     

Heat and mass transfer model of a ground heat exchanger: validation and sensitivity analysis

Validation of ground heat exchanger (GHE) model is presented using the experimental data obtained for both single and double pipe horizontal GHE. Sensitivity analysis of the GHE model shows the influence of the variation in the soil thermal conductivity, specific heat and density on the thermal performance of a GHE. Finally, the thermal performance of a GHE is analysed using both heat and mass transfer models, and conduction only model, as well as the influence of the initial soil moisture content on the thermal performance of a GHE. © 1998 John Wiley & Sons, Ltd.     

Numerical analysis reliability control for a double‐pipe heat exchanger with virtual entropy generation method

There are two primary laws including the first and second laws of thermodynamics that should be used to assess a process. Generally, only the first law of thermodynamics is investigated in numerical solutions, so it is possible to exist some numerical results that do not satisfy the second law of thermodynamics because of numerical errors. To achieve reliable numerical outcomes, it is better to apply two indexes of HEAT BALANCE ERROR and VIRTUAL ENTROPY GENERATION, which come from the second law of thermodynamics. In other words, an approach to develop computational fluid dynamics investigations is to take second law of thermodynamics into consideration. In this study, two different models including counterflow double‐pipe heat exchanger and single‐pipe with constant wall temperature are simulated in various cases with different efficiencies and temperature ratios. It is found that 46 cases of total 523 double‐pipe models and 24 cases of total 402 simulations of single‐pipe models had unacceptable results regarding to two mentioned criteria. The results revealed that it is less likely to gain unreliable results in smaller efficiency and lower inlet temperature for double‐pipe heat exchanger and single‐pipe respectively.     

Experimental investigation of heat transfer in a triple tube heat exchanger with coiled spring inserts

Experiments have been performed to investigate the effect of coiled spring inserts on heat transfer, pressure drop, and performance parameters of a triple tube heat exchanger (TTHX). Three different spring inserts having a pitch of 5, 10, and 15 mm are used and the diameter of the spring wire is taken as 1 mm. The experiments were carried out under a turbulent flow regime, with water as a working medium in parallel and counter flow configurations. The variation in different performance characteristics like heat transfer coefficient, Nusselt number, and effectiveness have been compared at various Reynolds numbers ranging between 4000 and 16,000 in the considered flow patterns. The Nusselt number of TTHX with the lowest pitch spring is found to be higher than that of the plain TTHX by 57.27% at Re = 4000 for the counter flow configuration. Both the thermal performance factor and effectiveness increased as the pitch of the spring insert was decreased. The effectiveness of TTHX with the lowest pitch spring insert is found higher than that of the plain TTHX by 43.84% in the counter flow pattern.     

通道内设置涡发生器时的强化传热研究现状与展望

随着强化传热技术的研究发展，各种形式的涡发生器的强化传热效果日益受到国内外的重视。文章比较全面地介绍了近年来国内外关于通道内布置各类涡发生器时的强化传热研究状况，并提出了有待进一步开展的研究内容。