An approximate method for transient behavior of finned-tube cross-flow heat exchangers

A closed, approximate, analytical approach is obtained for transient behavior of finned-tube, liquid/gas cross flow heat exchangers for the step change in the inlet temperature of the hot fluid. In this two-solid heat capacity approach, the heat capacities of the hot and cold fluids and the heat capacity of the wall and the fins are lumped separately. The temperature variation of both fluids between inlet and outlet is assumed to be linear. It is also assumed that flow rates and inlet conditions remain fixed for both fluids, except for the step change imposed on the inlet temperature of the hot fluid. Energy equations for the hot and cold fluids, fins and walls for both approaches are derived and solved analytically. The variation of the exit temperatures of both fluids with time are obtained for a step change in the inlet temperature of the hot fluid. The dynamic behavior of the heat exchanger is characterized by time constant, delay time and gain. This approach is easier to apply and can easily be modified for other heat exchanger types. Results are compared with the experimental and numerical results given in the literature.     

Parametric and internal study of the vortex tube using a CFD model

A computational fluid dynamics (CFD) model is used to investigate the energy separation mechanism and flow phenomena within a counter-flow vortex tube. A two-dimensional axi-symmetric CFD model has been developed that exhibits the general behavior expected from a vortex tube. The model results are compared to experimental data obtained from a laboratory vortex tube operated with room temperature compressed air. The CFD model is subsequently used to investigate the internal thermal-fluid processes that are responsible for the vortex tube's temperature separation behavior. The model shows that the vortex tube flow field can be divided into three regions that correspond to: flow that will eventually leave through the hot exit (hot flow region), flow that will eventually leave through the cold exit (cold flow region), and flow that is entrained within the device (re-circulating region). The underlying physical processes are studied by calculating the heat and work transfers through control surfaces defined by the streamlines that separate these regions. It was found that the energy separation exhibited by the vortex tube can be primarily explained by a work transfer caused by a torque produced by viscous shear acting on a rotating control surface that separates the cold flow region and the hot flow region. This work transfer is from the cold region to the hot region whereas the net heat transfer flows in the opposite direction and therefore tends to reduce the temperature separation effect. A parametric study of the effect of varying the diameter and length of the vortex tube is also presented.     

Experimental study and modelling of heat transfer during condensation of pure fluid and binary mixture on a bundle of horizontal finned tubes

An experimental investigation was conducted to measure the local heat transfer coefficient for each row in a trapezoidal finned horizontal tube bundle during condensation of both pure fluid (HFC 134a) and several compositions of the non-azeotropic binary mixture HFC 23/HFC 134a. The test section is a 13×3 (rows × columns) tube bundle and the heat transfer coefficient is measured using the modified Wilson plot method. The inlet vapour temperature is fixed at 40 °C and the water flow rate in each active row ranges from 170 to 600 l/h. The test series cover five different finned tubes all commercially available, K11 (11 fins/inch), K19 (19 fins/inch), K26 (26 fins/inch), K32 (32 fins/inch), K40 (40 fins/inch) and their performances were compared. The experimental results were checked against available models predicting the heat transfer coefficient during condensation of pure fluids on banks of finned tubes. Modelling of heat exchange during condensation of binary mixtures on bundles of finned tubes based on the curve condensation model is presented.     

Effect of heat conduction through the fins of a microchannel serpentine gas cooler of transcritical CO2 system

This paper presents results of an experimental study to investigate the effect of conduction through the fins on the capacity of a serpentine gas cooler. The gas cooler was a part of a transcritical CO₂ system which was operated in A/C mode. The capacity of the gas cooler was carefully measured in the chamber which simulated the outdoor condition with the original heat exchanger. In order to experimentally validate the conduction effect on the capacity, some sections of the fins, where the conduction was most significant, were cut by EDM (Electrical Discharge Machining). The capacity of the heat exchanger, after cutting fins, was measured in the same chamber at nearly identical test conditions as before cutting. Gas cooler capacity was improved up to 3.9% by cutting the fins, and temperature difference between refrigerant exit and air inlet for the gas cooler was reduced by 0.9–1.5 °C. The maximum uncertainty in the capacity measurements was 2.5% and the accuracy of temperature measurements was 0.1 °C. It was shown by system simulation that system COP could be improved by 5% by eliminating this severe conduction effect, as was done in this experiment. The tube surface temperature at some points of the gas cooler was measured and infrared images were taken to show the conduction effect before and after cutting fins.     

Experimental investigation of vortex tube refrigerator with a divergent hot tube

Energy separation performance of vortex tube can be improved by using a divergent hot tube. Experiments are carried out to investigate the influence of the geometrical parameters on vortex tube refrigeration capacity by using nitrogen as the working fluid. In this work, the parameters are focused on the divergence angle of hot tube, length of divergent hot tube and number of nozzle intakes. Experimental results present that there is an optimum angle for obtaining the highest refrigeration performance, and 4° is the optimal candidate under our experimental conditions. Divergent tube length which exceeds a critical length has slight effect on the refrigeration capacity. The critical length to diameter ratio is L/D = 12 in our experiment. Increasing number of nozzle intakes increases the sensitivity of temperature reduction and can obtain the highest possible temperature reduction. Moreover, similarity relations for the prediction of the cold exit stream are presented and confirmed by the experimental data.     

Investigation of turbulent heat transfer and nanofluid flow in a double pipe heat exchanger

In present study, heat transfer and turbulent flow of water/alumina nanofluid in a parallel as well as counter flow double pipe heat exchanger have been investigated. The governing equations have been solved using an in-house FORTRAN code, based on finite volume method. Single-phase and standard k-ε models have been used for nanofluid and turbulent modeling, respectively. The internal fluid has been considered as hot fluid (nanofluid) and the external fluid, cold fluid (base fluid). The effects of nanoparticles volume fraction, flow direction and Reynolds number on base fluid, nanofluid and wall temperatures, thermal efficiency, Nusselt number and convection heat transfer coefficient have been studied. The results indicated that increasing the nanoparticles volume fraction or Reynolds number causes enhancement of Nusselt number and convection heat transfer coefficient. Maximum rate of average Nusselt number and thermal efficiency enhancement are 32.7% and 30%, respectively. Also, by nanoparticles volume fraction increment, the outlet temperature of fluid and wall temperature increase. Study the minimum temperature in the solid wall of heat exchangers, it can be observed that the minimum temperature in counter flow has significantly reduced, compared to parallel flow. However, by increasing Reynolds number, the slope of thermal efficiency enhancement of heat exchanger gradually tends to a constant amount. This behavior is more obvious in parallel flow heat exchangers. Therefore, using of counter flow heat exchangers is recommended in higher Reynolds numbers.     

高效热交换器小温差换热过程热量不平衡问题分析

针对高效热交换器小温差换热过程进行了试验,结果表明:小温差换热过程冷、热侧热流量存在不平衡问题,冷、热侧热平衡误差随冷侧流体温升值和热侧流体的温降值的减小而增大.经分析发现,引起小温差换热过程热量不平衡的主要原因是随冷侧流体温升值和热侧流体的温降值的减小,温度测量仪表精度误差占比增大,导致热平衡误差增大.因此,建议测试...     

热端管长度对涡流管性能影响的实验研究

研制了不同热端管长度的涡流管,并以空气作为工作介质,通过实验研究了热端管长度对涡流管能量分离性能的影响.实验结果表明:对于常温涡流管,在入口压力为0.5MPa的情况下,相同冷流率时,随着热端管长度的增加,涡流管的制冷温度效应、单位制冷量和制冷系数增加,而其制热温度效应无显著的规律;对同一热端管长的涡流管,随着冷流率的增加,涡流管的制冷温度效应、单位制冷量和制冷系数增加,且在冷流率为40%～50%时出现峰值,而制热温度效应随冷流率的增加而增加,在冷流率范围内未出现峰值.     

冷端孔径对涡流管性能影响的实验研究

本文搭建了涡流管性能实验台,研究了不同冷端孔口直径的涡流管实验样机的性能。当进口压力为0.3~0.5 MPa时,分析了冷端孔径对冷端温降特性、制冷量特性、等熵温度效率特性及COP特性的影响。结果表明:冷端孔口直径对涡流管性能有很大影响,存在一个使涡流管冷端温降及制冷量均达到最大值的最佳冷端孔口直径,在本文设计的涡流管几何尺寸条件下,最佳冷端孔口直径为5 mm,最佳冷端孔口直径与热端直径比为0.5。     

Exergy Effectiveness Analysis of Three-Fluid Heat Exchanger

In this paper, exergy effectiveness is defined to describe the thermodynamic performance of three-fluid heat exchangers with three thermal communications. It is systemically discussed that the effects of eight non-dimensional design parameters, which include the ratio of the thermal resistances between fluids, the ratio of thermal capacity between fluids, the number of transfer units, the dimensionless inlet temperature of fluid, and the ratio of the inlet temperature of fluids on exergy effectiveness of three-fluid heat exchangers. Furthermore, the exergy effectiveness of the parallel flow is compared with that of the counter flow. The obtained results are helpful for engineers to evaluate and improve the performances of three-fluid heat exchangers.     

CFD simulation and experimental validation of a GM type double inlet pulse tube refrigerator

Pulse tube refrigerator has the advantages of long life and low vibration over the conventional cryocoolers, such as GM and stirling coolers because of the absence of moving parts in low temperature. This paper performs a three-dimensional computational fluid dynamic (CFD) simulation of a GM type double inlet pulse tube refrigerator (DIPTR) vertically aligned, operating under a variety of thermal boundary conditions. A commercial computational fluid dynamics (CFD) software package, Fluent 6.1 is used to model the oscillating flow inside a pulse tube refrigerator. The simulation represents fully coupled systems operating in steady-periodic mode. The externally imposed boundary conditions are sinusoidal pressure inlet by user defined function at one end of the tube and constant temperature or heat flux boundaries at the external walls of the cold-end heat exchangers. The experimental method to evaluate the optimum parameters of DIPTR is difficult. On the other hand, developing a computer code for CFD analysis is equally complex. The objectives of the present investigations are to ascertain the suitability of CFD based commercial package, Fluent for study of energy and fluid flow in DIPTR and to validate the CFD simulation results with available experimental data. The general results, such as the cool down behaviours of the system, phase relation between mass flow rate and pressure at cold end, the temperature profile along the wall of the cooler and refrigeration load are presented for different boundary conditions of the system. The results confirm that CFD based Fluent simulations are capable of elucidating complex periodic processes in DIPTR. The results also show that there is an excellent agreement between CFD simulation results and experimental results.     

Numerical analysis of evaporation performance in a finned-tube heat exchanger

This study discusses the effects of the heat exchanger type, refrigerant, inner tube configuration, and fin geometry on evaporator performance by adopting updated correlations of EVSIM, a numerical analysis model based on the tube-by-tube method developed by Domanski. The heat exchanger types considered are the cross-counter flow type and cross-parallel flow type. The refrigerants considered for the numerical test as a working fluid are R-134a, R-410A and R-22. For inner tube configuration, enhanced tube and smooth tube cases are considered. For the air side evaporation performance, heat exchangers using plate fins, wavy fins and slit fins are analyzed. Results show that the heat transfer rate of the cross-counter flow type heat exchanger is 3% higher than that of the cross-parallel flow type with R-22. The total heat transfer rate of the evaporator using R-410A is higher than those using R-22 and R-134a, while the total pressure drop of R-410A is lower than those of R-22 and R-134a. The heat transfer rate of the evaporator using enhanced tubes is two times higher than that using smooth tubes, but the pressure drop of the enhanced tube is 45–50% higher than that of the smooth tubes. The evaporation performance of slit fins is superior to that of plate fins by 54%.     

Effectiveness-NTU relations for parallel flow heat exchangers: The effect of kinetic energy variation and heat leak from outside

The effectiveness-NTU relations are valuable information for heat exchangers with fluids having significant variation in velocities in the presence of heat leak. In this paper, the closed-form relations for parallel-flow heat exchangers, when the heat leak is either on the cold or hot side of the heat exchanger in the presence of kinetic energy variation, are presented. The effectiveness was found to depend on NTU and fluid capacity ratio along with six other quantities that reflect the magnitude and axial distribution of the kinetic energy and heat leak on the hot and cold sides of the heat exchanger. The results are presented in a graphical form demonstrating the variation of effectiveness of the heat exchanger with the relevant parameters. It was demonstrated that in the limiting case, the solution reduces to the classical effectiveness-NTU relations for parallel-flow heat exchangers.     

带亲水层波纹翅片管换热器空气侧特性的参数影响分析

为了确定翅片间距、管排数和入口空气相对湿度等参数对析湿工况下带亲水层波纹翅片空气侧特性的影响,对7个带亲水层的波纹翅片管换热器进行了试验研究,并分别以Colbum换热因子和Fanning摩擦因子来反映空气侧换热和压降特性。结果表明,换热和摩擦因子都随着翅片间距的减小而增大。换热因子随着管排数的增加而减小,而摩擦因子受管排数的影响不显著。当入口相对湿度增大时,换热因子增大,而摩擦因子则几乎不变。入口水温越低,则入口空气相对湿度对换热因子的影响越显著。     

微重力环境低温流体无排气加注过程数值研究

针对加注系统受注贮箱,采用CFD方法就液氮贮箱无排气加注过程开展数值仿真,对比了不同重力水平下的无排气加注性能,分析了加注口结构、壁面初始温度、加注流体温度和加注流量等因素对微重力无排气加注性能的影响规律。所构建的二维轴对称模型将流体区与固壁区一起作为求解区域并划分网格,并通过植入用户自定义程序(UDF)计算加注口液体闪蒸过程及气液之间的热质交换。经过实验数据验证,该模型能够合理展示箱内温度场分布和相分布情况,并获得贮箱压力等参数变化信息。数值计算结果表明:(1)加注条件相同时,微重力工况较常重力工况体现出更好的无排气加注性能。(2)微重力条件下,无排气加注性能几乎不受加注口结构的影响,壁面初始温度和加注流体温度越高,贮箱压力越高,加注流量仅对加注时间有显著影响。     

Experimental Study of Heat Transfer Enhancement in Solar Tower Receiver Using Internal Fins

The receiver is an important element in solar energy plants. The principal receiver’s tubes in power plants are devised to work under extremely severe conditions, including excessive heat fluxes. Half of the tube’s circumference is heated whilst the other half is insulated. This study aims to improve the heat transfer process and reinforce the tubes’ structure by designing a new receiver; by including longitudinal fins of triangular, circular and square shapes. The research is conducted experimentally using Reynolds numbers ranging from 28,000 to 78,000. Triangular fins have demonstrated the best improvement for heat transfer. For Reynolds number value near 43,000 Nusselt number (Nu) is higher by 3.5% and 7.5%, sequentially, compared to circular and square tube fins, but varies up to 6.5% near Re = 61000. The lowest friction factor is seen in a triangular fin receiver; where it deviates from circular fins by 4.6%, and square fin tubes by 3.2%. Adding fins makes the temperature decrease gradually, and in the case of no fins, the temperature gradient between the hot tube and water drops sharply in the planed tube by 7%.     

Shell side direct expansion evaporation of ammonia on a plain tube bundle with inlet quality effect in the presence of exit superheat

In the present study, tests were performed to investigate the direct expansion evaporation characteristics of ammonia on shell side of triangular pitch plain tube bundle at saturation temperature of −1.7 °C and −20 °C, with heat flux ranging from 5 to 45 kW m⁻², and inlet quality from 0 to 30% with exit degree of superheat ranging from 2 to 10 °C. The results suggest that heat transfer coefficient increases with both saturation temperature and heat flux and decreases with degree of exit superheat. The inlet quality effects were more significant at higher saturation temperature than at lower saturation temperature. A correlation was developed for outside boiling of ammonia on plain tube bundle in direct expansion mode with inlet vapor quality and compared with previous pool boiling and bundle correlations with ammonia as a refrigerant.     

CYRANO: a computational model for the detailed design of plate-fin-and-tube heat exchangers using pure and mixed refrigerants

A computational model for the detailed design of finned coils has been developed. This programme discretises heat exchangers into tube elements for which the governing equations are solved using local values of temperature, pressure, physical properties and heat transfer coefficients. Single-phase, condenser and evaporator cases can be automatically treated using water, R22, R134a, and refrigerant mixtures based on R32, R125, and R134a. The software can handle non-conventional coil circuits with different numbers of inlets and outlets, non-uniform air distribution at the coil inlet face, using smooth, wavy and louvered fins, and smooth and internally finned tubes. The programme has been validated on seven finned coils using pure fluids, with and without moisture condensation on the fins. Comparisons with tests show errors of less than 5% on the coil duty and of the order of 30% on the refrigerant pressure drop. A performance simulation of a coil using R22 and a ternary mixture is presented to validate the programme algorithms developed for mixtures.     

Time behavior of heat fluxes in thermally coupled turbulent dispersed particle flows

The effect on heat transfer produced by injection of solid microparticles with high thermal capacity in turbulent channel flow is analyzed. Convection is forced by letting the fluid flow between a hot plate and a cold plate under zero-gravity conditions. An Eulerian?CLagrangian approach based on direct numerical simulation of turbulence (shear Reynolds number Re*?=?150 and molecular Prandtl number Pr?=?3) and on point-particle tracking is used. Full momentum and energy coupling between fluid and particles is considered. Different particle sizes and different particle concentrations are examined.     

Numerical analysis on the heat and work transfer due to shear in a hot cascade Ranque–Hilsch vortex tube

Cascading of vortex tubes is a possible implementation to extract significantly larger amount of useful work. A hot cascade-type RHVT makes use of the cold gas for cooling purposes while improving the heating capacity of the hot gas. In a vortex tube inflow pressure is the only source of energy which converts into thermal energy. The conversion of pressure energy into thermal energy is associated with the heat and work transfer due to shear along the radial, axial and tangential directions. In this paper, the physics of fluid flow and thermal separation are studied based on the heat and work transfer due to shear along all three directions. The work transfer due to the action of tangential shear is always from the cold to hot fluid layers and is the most dominant factor in the thermal separation process. The contribution increases considerably with hot cascading. However, the process of thermal separation degrades due to the effect of sensible heat transfer.