Experimental Study on Air Cooling Via a Multiport Mesochannel Cross-Flow Heat Exchanger

The air-side heat transfer and flow characteristics of cross-flow multiport slab mesochannel heat exchanger are investigated experimentally in this article. The multiport slab mesochannel heat exchanger consists of 15 finned aluminum slabs; each slab contains 68 flow channels of 1 mm circular diameter. The cold deionized water at a constant mass flow rate was forced to flow through the mesochannels, whereas the hot air at different velocities was allowed to pass through the finned passages of the heat exchanger core in cross-flow orientation. The heat transfer and fluid flow key parameters were examined in the region of the air-side Reynolds number in the range of 972–2758, with a constant water-side Reynolds number of 135. The effect of air-side Reynolds number on air-side Nusselt number was examined and a general correlation of Nusselt number with Reynolds number was obtained. The Nusselt number value was found to be higher in comparison with other research works for the corresponding Reynolds number range. The multiport mesochannel flat slab geometry has offered uniform temperature distribution into the core. This uniform temperature distribution leads to higher heat transfer over stand-alone inline flow tube bank.     

Thermal performance and pressure drop of the helical-coil heat exchangers with and without helically crimped fins

In the present study, the thermal performance and pressure drop of the helical-coil heat exchanger with and without helical crimped fins are studied. The heat exchanger consists of a shell and helically coiled tube unit with two different coil diameters. Each coil is fabricated by bending a 9.50 mm diameter straight copper tube into a helical-coil tube of thirteen turns. Cold and hot water are used as working fluids in shell side and tube side, respectively. The experiments are done at the cold and hot water mass flow rates ranging between 0.10 and 0.22 kg/s, and between 0.02 and 0.12 kg/s, respectively. The inlet temperatures of cold and hot water are between 15 and 25 °C, and between 35 and 45 °C, respectively. The cold water entering the heat exchanger at the outer channel flows across the helical tube and flows out at the inner channel. The hot water enters the heat exchanger at the inner helical-coil tube and flows along the helical tube. The effects of the inlet conditions of both working fluids flowing through the test section on the heat transfer characteristics are discussed.     

Single-phase heat transfer and pressure drop in the micro-fin tubes with coiled wire insert

The heat transfer characteristics and the pressure drop of the horizontal double pipes with and without coiled wire insert are investigated. The inner and outer diameters of the micro-fin tube are 8.92 and 9.52 mm, respectively. The coiled wire is fabricated by bending a 1-mm-diameter iron wire into the coil wire with coil diameter of 7.80 mm. Cold and hot water are used as working fluids in shell side and tube side, respectively. The test runs are performed at the cold and hot water mass flow rates ranging between 0.01 and 0.07 kg/s and between 0.04 and 0.08 kg/s, respectively. The inlet cold and hot water temperatures are between 15 and 20 °C and between 40 and 45 °C, respectively. The results obtained from the micro-fin tube with coiled wire insert are compared with those obtained from the smooth and micro-fin tubes.     

Heat transfer characteristics of a temperature-dependent-property fluid in shell and coiled tube heat exchangers

An experimental investigation was performed to study the heat transfer characteristics of temperature-dependent-property engine-oil inside shell and coiled tube heat exchangers. For this purpose, a well-instrumented set-up was designed and constructed. Three heat exchangers with different coil pitches were selected as the test section for counter-flow configuration. Engine-oil was circulated inside the inner coiled tube, while coolant water flowed in the shell. All the required parameters like inlet and outlet temperatures of tube-side and shell-side fluids, flow rate of fluids, etc were measured using appropriate instruments. An empirical correlation existed in the previous literature for evaluating the shell-side Nusselt number was invoked to calculate the heat transfer coefficients of the temperature-dependent-property fluid flowing in the tube-side of the heat exchangers. Using the data of the present study, an empirical correlation was developed to predict the heat transfer coefficients of the temperature-dependent-property fluid flowing inside the shell and coiled tube heat exchangers.     

Air side heat transfer characteristics of plate finned tube heat exchangers with slit fin configuration under wet conditions

An experimental study was performed on compact finned tube heat exchangers under wet conditions. Eight different finned tube heat exchangers having slit fins with hydrophilic coatings were tested. The effects of tube diameter, the number of tube rows, and inlet air relative humidity on air side heat transfer and pressure drop characteristics were investigated. Air side heat transfer coefficients were calculated using the log mean enthalpy difference method. The effects of the number of tube rows and the tube diameter on the Colburn j-factor and the f-factor were larger compared with those of the inlet air relative humidity. The Colburn j-factor and the f-factor of the single-row heat exchanger were larger than those of two- or three-row heat exchangers. The j-factor for the 5.30 mm tube diameter was compared with those for 7.35 mm and 9.95 mm tube diameters at 46% RH and was found to be 33% and 55% larger, respectively.     

Effect of Surface Tension and Evaporation on Phase Change of Fuel Droplets

Because of the low heat transfer coefficient of gases, it is necessary to use extended surfaces on the gas side when exchanging heat with liquids or two-phase fluids. Such finned tube exchangers are a very important basic class of heat exchangers. Although finned tube exchangers have been in existence for 50 yr, there have been numerous developments in the technology. This paper attempts to survey the recent developments related to the air-side aspects of heat exchanger design. Nearly all of the recent developments have been empirical because of the complicated gas-side flow structure. Among the developments discussed are the use of special surface geometries for enhanced heat transfer, row effects, local heat transfer coefficient distribution, and correlations for heat transfer coefficient and pressure drop. Two important design configurations—circular finned tubes and plate fin designs—are considered separately.     

Experimental Investigation on Flow Boiling Heat Transfer and Pressure Drop of HFC-134a inside a Vertical Helically Coiled Tube

An investigation on flow boiling heat transfer and pressure drop of HFC-134a inside a vertical helically coiled concentric tube-in-tube heat exchanger has been experimentally carried out. The test section is a six-turn helically coiled tube with 5.786-m length, in which refrigerant HFC-134a flowing inside the inner tube is heated by the water flowing in the annulus. The diameter and the pitch of the coil are 305 mm and 45 mm, respectively. The outer diameter of the inner tube and its thickness are respectively 9.52 and 0.62 mm. The inner diameter of the outer tube is 29 mm. The average vapor qualities in test section were varied from 0.1 to 0.8. The tests were conducted with three different mass velocities of 112, 132, and 152 kg/m²-s. Analysis of obtained data showed that increasing of both the vapor qualities and the mass fluxes leads to higher heat transfer coefficients and pressure drops. Also, it was observed that the heat transfer coefficient is enhanced and also the pressure drop is increased when a helically coiled tube is used instead of a straight tube. Based on the present experimental results, a correlation was developed to predict the flow boiling heat transfer coefficient in vertical helically coiled tubes.     

Air-side heat transfer and friction characteristics of biofouled evaporator under wet conditions

The effects of biofouling on air-side heat transfer and friction characteristics under wet conditions of three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were investigated experimentally. Experimental results indicate that the biofouled fin efficiency of the evaporator decreases by 15.5% compared with the clean evaporator under the condition of the biofouled area ratio of 60% at the inlet air velocity of 2.0 m/s; The ranges of friction fouling factor and heat transfer fouling factor are 19.8%–43.1% and −15.6%−13.1%, respectively; a small quantity of biofouled particles can enhance heat transfer at low Reynolds number, and the enhancement effect decreases with the increase of Reynolds number.     

Experimental investigation of shell and coiled tube heat exchangers using wilson plots

In this work, an experimental investigation was performed to study the shell and helically coiled tube heat exchangers. Three heat exchangers with different coil pitches and curvature ratios were tested for both parallel-flow and counter-flow configurations. All the required parameters like inlet and outlet temperatures of tube-side and shell-side fluids, flow rate of fluids, etc. were measured using appropriate instruments. Overall heat transfer coefficients of the heat exchangers were calculated using Wilson plots. Heat transfer coefficients of shell and tube sides were evaluated invoking the calculated overall heat transfer coefficients. The inner Nusselt numbers were compared to the values existed in open literature. Though the boundary conditions were different, a reasonable agreement was observed.     

Effect of coil-wire insert on heat transfer enhancement and pressure drop of the horizontal concentric tubes

In the present study, the heat transfer characteristics and the pressure drop of the horizontal double pipe with coil-wire insert are investigated. The inner and outer diameters of the inner tube are 8.92 and 9.52 mm, respectively. The coiled wire is fabricated by bending a 1 mm diameter of the iron wire into a coil with a coil diameter of 7.80 mm. Cold and hot water are used as working fluids in the shell side and tube side, respectively. The test runs are performed at the cold and hot water mass flow rates ranging between 0.01 and 0.07 kg/s, and between 0.04 and 0.08 kg/s, respectively. The inlet cold and hot water temperatures are between 15 and 20 °C, and between 40 and 45 °C, respectively. The effect of the coil pitch and relevant parameters on heat transfer characteristics and pressure drop are considered. Coil-wire insert has significant effect on the enhancement of heat transfer especially on laminar flow region. Non-isothermal correlations for the heat transfer coefficient and friction factor are proposed. There is reasonable agreement between the measured data and predicted results.     

内展翅片换热器应用在压缩空气系统中的节能效益分析

文中通过实验数据分析以及实际工程案例,就内展翅片换热器在压缩空气系统的相同工况下,采用光管换热器和外翅片换热器的换热效果及阻力特性进行了比较,结果表明,内展翅片换热器不仅具有较好的强化传热效果,而且单位压降损耗低,节能效果显著。     

Effect of curvature ratios on the heat transfer and flow developments in the horizontal spirally coiled tubes

Effect of curvature ratios on the heat transfer and flow developments in the horizontal spirally coiled tubes are investigated. The spirally coiled tube is fabricated by bending a 8.00 mm diameter straight copper tube into a spiral-coil of five turns. The spirally coiled tube with three different curvature ratios of 0.02, 0.04, 0.05 under constant wall temperature are tested. Cold water entering the innermost turn flows along the spiral tube and flows out at the outermost turn. The turbulent flow and heat transfer developments are simulated by using the k–ε standard turbulence model. A finite volume method with an unstructured nonuniform grid system is employed for solving the model. The simulated results are validated by comparing with the present experiment. The predicted results for the convective heat transfer and flow characteristics are reasonable agreement with the experiments. The centrifugal force has significant effect on the enhancements of heat transfer and pressure drop. In addition, due to this force, the heat transfer and pressure drop obtained from the spirally coiled tube are higher than those from the straight tube.     

内展翅片换热器在压缩空气系统中的应用分析

通过实验数据分析以及实际工程案例,对内展翅片换热器在压缩空气系统的相同工况下采用光管换热器和外翅片换热器的换热效果及阻力特性进行了比较。结果表明,内展翅片换热器不仅具有较好的强化传热效果,而且单位压降损耗低,节能效果显著。     

Air-Side Performance Analysis of a Wire-on-Tube Heat Exchanger With an Oscillating Heat Pipe as an Extended Surface Under Natural Convection Conditions

This study investigates the performance of a wire-on-tube heat exchanger that uses an oscillating heat pipe as an extended surface under natural convection. The results have been compared with those of a conventional wire-on-tube unit. The heat exchanger exchanges heat between hot water flowing inside the tube, which is varied from 50 to 85°C, and the surrounding ambient air, which is kept constant at 25°C. The tested unit is installed in the horizontal and vertical directions, and the effects of tube diameter and tube and wire pitches on thermal performance are considered. The results show that the performance of the heat exchanger with an oscillating heat pipe is slightly higher than that of the conventional wire-on-tube heat exchanger. It was also found that the air-side performance of the heat exchanger is directly proportional to the tube diameter and tube and wire pitches. Moreover, when the heat exchanger is installed horizontally, its performance is approximately 15–20% higher than that of the unit in the vertical installation. A heat transfer model for evaluating the heat exchanger performance is also developed, and the results agree well with the experimental data.     

Condensation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger

The two-phase heat transfer coefficient and pressure drop of pure HFC-134a condensing inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786 m long helically coiled double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth copper tubing of 9.52 mm outer diameter and 8.3 mm inner diameter. The outer tube is made from smooth copper tubing of 23.2 mm outer diameter and 21.2 mm inner diameter. The heat exchanger is fabricated by bending a straight copper double-concentric tube into a helical coil of six turns. The diameter of coil is 305 mm. The pitch of coil is 35 mm. The test runs are done at average saturation condensing temperatures ranging between 40 and 50 °C. The mass fluxes are between 400 and 800 kg m⁻² s⁻¹ and the heat fluxes are between 5 and 10 kW m⁻². The pressure drop across the test section is directly measured by a differential pressure transducer. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy rejected from the test section. The effects of heat flux, mass flux and, condensation temperature on the heat transfer coefficients and pressure drop are also discussed. It is found that the percentage increase of the average heat transfer coefficient and the pressure drop of the helically coiled concentric tube-in-tube heat exchanger, compared with that of the straight tube-in-tube heat exchanger, are in the range of 33–53% and 29–46%, respectively. New correlations for the condensation heat transfer coefficient and pressure drop are proposed for practical applications.     

Air-side heat transfer and friction characteristics of biofouled evaporator under wet conditions

The effects of biofouling on air-side heat transfer and friction characteristics under wet conditions of three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were investigated experimentally. Experimental results indicate that the biofouled fin efficiency of the evaporator decreases by 15.5% compared with the clean evaporator under the condition of the biofouled area ratio of 60% at the inlet air velocity of 2.0m/s; The ranges of friction fouling factor and heat transfer fouling factor are 19.8%―43.1% and ―15.6%―13.1%, respectively; a small quantity of biofouled particles can enhance heat transfer at low Reynolds number, and the enhancement effect decreases with the increase of Reynolds number.     

An experimental study of thermal performance of shell-and-coil heat exchangers

In the present study an experimental investigation of the mixed convection heat transfer in a coil-in-shell heat exchanger is reported for various Reynolds and Rayleigh numbers, various tube-to-coil diameter ratios and dimensionless coil pitch. The purpose of this article is to assess the influence of the tube diameter, coil pitch, shell-side and tube-side mass flow rate over the performance coefficient and modified effectiveness of vertical helical coiled tube heat exchangers. The calculations have been performed for the steady-state and the experiments were conducted for both laminar and turbulent flow inside coil. It was found that the mass flow rate of tube-side to shell-side ratio was effective on the axial temperature profiles of heat exchanger. The results also indicate that the ? − NTU relation of the mixed convection heat exchangers was the same as that of a pure counter-flow heat exchanger.     

Mathematical Model for Predicting the Heat Transfer Characteristics of a Helical-Coiled,Crimped, Spiral,Finned-Tube Heat Exchanger

This paper is a continuation of the authors’ previous work. Theoretical and experimental studies of the heat transfer characteristics of a helical-coiled, crimped, spiral, finned-tube heat exchanger in dry surface conditions are presented. The test section is a helical-coiled, finned-tube heat exchanger. The coil unit is composed of four concentric helical-coiled tubes of different diameters. All tubes are constructed by bending straight copper tube into seven layers of helical coil. Aluminum crimped spiral fins, with an outer diameter of 28.25 mm and a thickness of 0.5 mm, are connected around the tube. Hot water is used as a working fluid for the tube side, while ambient air is used for the shell side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. A mathematical model is developed and the simulation results show reasonable agreement with the experimental data.     

Experimental Investigation of Heat Transfer and Resistance Characteristics of a Finned Oval-Tube Heat Exchanger With Different Air Inlet Angles

The air inlet flow direction is not orthogonal to the heat exchanger surface in many cases. To study the performance of the heat transfer and pressure drop of a heat exchanger with different air inlet angles, this paper shows the experimental system about a finned oval-tube heat exchanger inclined toward the air incoming flow direction. The heat transfer and pressure drop characteristics of four air inlet angles (90°, 60°, 45°, and 30°) are studied separately for the Reynolds number ranging from 1300 to 13000 in this study. The experimental correlations of Nusselt number and resistance coefficient of the air side are acquired. The results show that the overall heat transfer coefficients become smaller and smaller with the decrease of the air inlet angles, while the pressure drops have significant changes. The heat transfer performances of the heat exchanger under the three inclined air inlet angles are worse than that at 90°. Among the three inclined angles, the performance at 45° is the best under identical mass flow rate criterion and at low Reynolds number under identical pressure drop criterion; that at 60° is the best at large Reynolds under identical pressure drop criterion. Finally, some conclusions are attained about the effects of the air inlet angles on the heat transfer and pressure drop performance of the finned oval-tube heat exchanger.     

不同纵向涡发生器流动传热的数值模拟

利用三维数值模拟的方法对带有3种异形纵向涡发生器的H型翅片椭圆管换热器的空气侧流动传热特性进行研究。基于H型翅片椭圆管束,讨论了在不同雷诺数下,纵向涡发生器的摆放位置、摆放攻角和形状对空气侧流动传热的影响。研究表明:纵向涡发生器能够将高能量的流体引向流速较低的壁面区域,使冷热流体之间的混合加剧,增强流体的湍流动能,进而达到强化传热的效果;与无纵向涡发生器的管束相比,带纵向涡发生器管束的传热效果有明显的提高;当纵向涡发生器后置时,换热器的传热效果最优;在雷诺数相同,攻角为30°时,流体的传热性能和阻力特性均达到最优;相同攻角摆放时,椭圆角矩形发生器的传热性能和阻力因子均优于其他两种形式的发生器。研究结果为烟气余热回收系统换热器传热性能强化提供理论依据。