Study on the heat-flow controllable heat exchanger (1st report): Thermal characteristics of heat-flow controllable heat exchanger

To regulate temperature in passive solar houses and green houses, the authors have developed a heat exchanger capable of controlling the heat flow. It has a thermal switch mechanism without mechanically moving parts. It consists of an evaporator, a condenser, a vapor passage pipe, a liquid return pipe having an inverted-U-pipe, and a heater mounted on the inverted-U-pipe. The heat exchanger can transfer, or reduce to zero, heat from the evaporator to the condenser by regulating a slight heater input. The authors have fabricated a model of the heat-flow controllable heat exchanger to examine its thermal switching and heat exchange characteristics, and then compared the obtained results with calculation results. It was clarified that the experimental results agree with the calculation results.     

Evaporation performance of a new plate‐type heat exchanger with winding tubes on plate surfaces (estimation of performance of the heat exchanger)

A method for evaluating and predicting the performance of a newly developed plate‐type heat exchanger as an evaporator for water‐refrigerant systems such as chillers has been developed. The main component of the developed heat exchanger consists of plates packed together in a casing with winding tubes connected to both sides of the plates. Refrigerant flows inside the tubes, and water flows in the space between the plates. A herringbone‐like pattern is formed in this space by the cross sections of the winding tubes. The newly developed method estimates evaporation performance of the developed heat exchanger using new empirical correlations. There are correlations for heat transfer and pressure drop in winding‐tube banks on the water side, and correlation for the pressure drop on the refrigerant side. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(4): 245–257, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20009     

Forced convective heat transfer in uniformly heated horizontal tubes (2nd report, theoretical study)

Effects of buoyancy force on forced laminar convective heat transfer in a uniformly heated horizontal tube may not be neglected at large Re Ra. This 2nd report deals with a theoretical investigation of this problem on a fully developed laminar flow and compares the results with experimental results reported in the 1st report.

In order to back up assumptions made in the following analysis, patterns of secondary flow due to buoyancy are observed in flow visualization experiments. An approximate solution for very large Re Ra is obtained. Nusselt numbers are shown as a function of Re Ra and Pr and are shown to be in fairly good agreement with experimental results on air. Resistance coefficients are also obtained as a function of Re Ra and Pr.     

Heat transfer characteristics of a reservoir embedded loop heat pipe (2nd report,Influence of noncondensable gas on heat transfer characteristics)

High‐powered satellites need larger heat rejection areas. A deployable radiator is one of the key technologies for a high‐powered satellite bus. A Reservoir Embedded Loop Heat Pipe (RELHP) is a two‐phase heat transfer device that constitutes the deployable radiator. RELHP has an evaporator core which is used as a liquid reservoir to enhance operational reliability. For use on satellites, RELHP is required to have a lifetime greater than 10 years. In the case of conventional heat pipes, it is generally known that noncondensable gas (NCG) has worse heat transport characteristics. On the other hand, the influence of NCG on a RELHP is not still obvious. This paper presents the heat transport characteristics of RELHP for the case of changing NCG volume by experiment and calculation. It was found that NCG increases temperature rise at the evaporator. NCG volume in a RELHP has a great influence on heat transport characteristics due to the reservoir pressure increase caused by NCG. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 459–473, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20181     

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     

Premature transition to film boiling with stepwise heat generation 2nd report: Effect of wall material and surface condition

The effects of wall material and surface condition on the behavior of an initial boiling bubble of R113 subjected to transient heating were investigated using a heater with a large heat capacity. The behavior of the initial bubble is closely related to premature transition to film boiling of liquids with high wettability. An initial bubble, which is peculiarly shaped like a “straw hat” and leads to premature transition in saturated liquid nitrogen (as reported in a previous paper), also appears on a heated wall with large heat capacity and grows rapidly to cover the entire wall surface. From the observations using a high-speed video camera, the initial bubble is found to be a coalesced bubble into which small bubbles activated in succession along the heated surface are rolled. The growth rate of the initial bubble along the heated surface is not greatly affected by the thermal conductivity of the wall material but is affected markedly by the surface roughness. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res. 25(1): 51–63, 1996     

Study on forced convective heat transfer in curved pipes: (1st report,laminar region)

The theoretical study of fully developed laminar flow in a curved pipe gives a satisfactory result for resistance coefficient and heat-transfer coefficient in fully developed temperature field under the condition of uniform heat flux at large Dean numbers [= Re√(a/R)]. The ratio of the Nusselt number for a curved pipe flow to that for a straight pipe flow is obtained as a function of Dean number and Prandtl number. As Dean number and Prandtl number increase, the effect of curvature on flow resistance and heat transfer increases, but the Nusselt number ratio approaches to the asymptotic value with the increasing Prandtl number. Experimental study is carried out for air flow in a curved pipe. The velocity and temperature distributions are measured and the Nusselt number ratio obtained by the experiments is shown to be in good agreement with that of the theoretical analysis.     

Natural convection heat transfer in an anisotropic porous cavity heated from side (2nd report,experiment using a Hele‐Shaw cell)

Natural convection heat transfer and flow structure in an anisotropic porous medium of square cavity saturated with a Boussinesq fluid has been studied experimentally using a Hele‐Shaw cell. The permeability ratio defined by K = K_y/K_x was set to three different values: 0.4, 1, and 2.5. The convection patterns at the three permeability ratios are visualized at several different Rayleigh numbers by a pH indicator method. When K is 0.25, the visualized flow is mainly in the vertical direction. On the contrary, for K = 4 the convecting flow is in the horizontal direction. The average heat transfer coefficients are also measured, and the corresponding Nusselt numbers are plotted as a function of K. It is found that the corresponding Nusselt numbers are correlated with (KRa)^1/2. The experimental results of the flow pattern and heat transfer are in good agreement with those obtained by our previous theory. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(6): 463–474, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10046     

Prediction of heat exchanger capacity by thermal network method (Influence of thermal conduction in fins on capacity of a condenser)

This paper describes the influence of heat flow from high‐temperature refrigerant to low‐temperature refrigerant through fins by thermal conduction. To estimate that influence, we applied a thermal network method that can consider refrigerant quality distribution in the heat exchanger. At the same time, for verifying the estimation, an experiment was performed with a two‐row, two‐pass heat exchanger. Prediction shows that the heat transfer capacity of a condenser is reduced by 3% for a simple two‐row, two‐pass heat exchanger by heat conduction in fins. Comparison of experimental results and predicted results proves that the prediction error was within 1% for condenser capacity. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(2): 101–114, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20184     

Evaluation of the performance of a ground-source heat-pump system with series GHE (ground heat exchanger) in the cold climate region

Ground-source heat-pump systems provide a new and clean way of heating buildings in the world. They make use of renewable energy stored in the ground, providing one of the most energy-efficient ways of heating buildings. Consumption costs are lowered through the use of free energy from the environment, and the dependence on fossil fuels simultaneously reduces. The aim of this study is to evaluate the performance of vertical ground-source heat-pump system for climatic condition of Erzurum having cold climate in Turkey. For this purpose, an experimental set-up was constructed. The experimental apparatus consisted of a series GHE (ground heat exchanger), a liquid-to-liquid vapor compression heat pump, water circulating pumps and other measurement equipments. In this study, the performance of the system was experimentally investigated. The experimental results were obtained from October to May for the months of heating season of 2008–2009. The experimental results indicate that the average heat-pump COP and overall system’s COPS values are approximately 3.0 and 2.6 in the coldest months of heating season. This study also shows that this system could be used for residential heating in the province of Erzurum being a cold climate region of Turkey.     

Enhancement of condensation on a vertical plate (2nd report,prediction of condensation on a dispersed finned surface)

In this study, a prediction model for condensation heat transfer on a vertical dispersed finned surface was proposed, utilizing the Adamek‐Webb model for condensation heat transfer outside a horizontal finned tube. The prediction model was based on two main experimental observation results. One is the phenomena of the condensate retention at the bottom of each row of the dispersed fin. Another is the offset phenomena of the condensate flow between each row of the dispersed fin. Given the results by the present model, it is predicted that the dependence of the condensation heat transfer coefficient for the dispersed finned surface on the fin pitch is controlled mainly by the dispersed fin length, not the total fin length. On the contrary, for a different fin pitch, the effect to the condensation heat transfer by dispersing the fin is different. From comparison with the experiment results, it is confirmed that the present model was able to predict the condensation with extremely good precision when the fin pitch was larger. Further, when the fin pitch was smaller, the predicted values were higher than the experimental values, but the tendency of the condensation heat transfer with dispersing the fin was nearly predicted. In addition, this condensing model can predict the experimental values with an error of 25% at the maximum in a range of fin pitch 0.6 mm to 1 mm. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20288     

Study on heat transport of an osmotic heat pipe: Part 2. Flow in the membrane module

An osmotic heat pipe is a top‐heat‐mode heat pipe in which a heat transport medium flowing down is pumped up by means of osmosis in the membrane module. The osmosis is dependent on the concentration difference between the solution at the inside surface and the solvent at the outside surface of the membrane. In addition, convection in the solution inside the membrane affects the concentration of the solution in contact with the inside surface. Thus, the concentration and solution flow rate greatly affect the osmotic pumping rate and the heat transport rate. Therefore, in the present study, the flow in the membrane module was investigated in detail. Using the ratio of the concentration at the inside surface to the mixed mean concentration at the corresponding site, relations for these concentrations and the solution flow rate along the solution channel in the membrane module are derived semitheoretically. These relations can be used to correlate experimental data to within a ±20% error. In addition, a method for increasing the osmotic pumping rate and the heat transport rate of the osmotic heat pipe is proposed. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 317–332, 2000     

Experimental study on the effect of porous medium on performance of a single tube heat exchanger: A CO2 case study

An experimental study on single‐phase laminar forced convection in a single porous tube heat exchanger is presented. Parametric studies are conducted for different inlet pressures, different mass flow rates, and different porosities to evaluate the effects of particle diameter and Reynolds number on the heat transfer and friction factor. The Nusselt number and friction factor are developed for efficient design of a porous heat exchanger based on the present configuration. Heat is transferred to the walls of the heat exchanger by natural convection mode. Gravel sand with different porosities is used as a porous medium during the tests. The flow of carbon dioxide as a working fluid in the porous medium is modeled using the Brinkman–Forchheimer‐extended Darcy model. A dimensionless performance parameter is developed in order to be used in evaluating the porous tube heat exchanger based on both the heat transfer enhancement and the associated pressure drop. The study covers a wide range of inlet pressures (P_i), mass flow rates ( ), porosity of gravel sand (ε), and particle diameters (d_m) which ranged 34.5 ≤ P_i ≤ 43 bars, 8 ?? 10^?5 ≤ ≤ 16 ?? 10^?5 kg/s, 34.9% ≤ ε ≤ 44.5%, 1.25 ≤ d_m ≤ 5.15 mm, respectively. This study revealed that a smaller particle diameter can be used to achieve higher heat transfer enhancement, but a larger particle diameter leads to a more efficient performance based on heat transfer enhancement. The average heat transfer coefficient of carbon dioxide decreases when the porosity increases. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21059     

Study on heat transfer characteristics of reservoir embedded loop heat pipe (1st report,Influence of evaporator orientation against gravity and charged liquid weight on heat transfer characteristics)

High‐powered satellites need larger heat rejection areas. A deployable radiator is one of the key technologies for a high‐powered satellite bus. A Reservoir Embedded Loop Heat Pipe (RELHP) is a two‐phase heat transfer device that constitutes a deployable radiator. RELHP has an evaporator core which is used as a liquid reservoir to enhance operational reliability. This paper presents the heat transport characteristics of a RELHP under changing evaporator orientation against gravity and charged ammonia weight by experiment and calculation. Liquid slug position in the reservoir has a great influence on heat transport characteristics, caused by changing heat transfer coefficients between returned liquid into the evaporator and vapor in the reservoir. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(3): 143– 157, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20150     

Numerical investigation on the single phase forced convection heat transfer characteristics of TiO2 nanofluids in a double-tube counter flow heat exchanger

In this study, forced convection flows of nanofluids consisting of water with TiO₂ and Al₂O₃ nanoparticles in a horizontal tube with constant wall temperature are investigated numerically. The horizontal test section is modeled and solved using a CFD program. Palm et al.'s correlations are used to determine the nanofluid properties. A single-phase model having two-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the nanofluid flow. The numerical investigation is performed for a constant particle size of Al₂O₃ as a case study after the validation of its model by means of the experimental data of Duangthongsuk and Wongwises with TiO₂ nanoparticles. The velocity and temperature vectors are presented in the entrance and fully developed region. The variations of the fluid temperature, local heat transfer coefficient and pressure drop along tube length are shown in the paper. Effects of nanoparticles concentration and Reynolds number on the wall shear stress, Nusselt number, heat transfer coefficient and pressure drop are presented. Numerical results show the heat transfer enhancement due to presence of the nanoparticles in the fluid in accordance with the results of the experimental study used for the validation process of the numerical model.     

Heat transfer and pressure loss characteristics of a heat exchanger for recovering latent heat (effects of straight fin length and fin pitch on the heat transfer and pressure loss characteristics)

In recent years, the requirement for the 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 for improving thermal efficiency by recovering latent heat. This paper describes the heat transfer and pressure loss characteristics of a latent heat recovery type heat exchanger having straight fins (fin length: 65 mm or 100 mm, fin pitch: 2.5 mm or 4 mm). These were clarified by measuring the exchange heat quantity, the pressure loss of the heat exchanger, and the heat transfer coefficient between the outer fin surface and gas. The effects of fin length and fin pitch on heat transfer and pressure loss characteristics were clarified. Furthermore, equations for predicting the heat transfer coefficient and pressure loss which are necessary for heat exchanger design were proposed. ©2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(4): 230– 247, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20153 Copyright © 2004 Wiley Periodicals, Inc.     

A simple method for predicting the performance of a heat exchanger mounted in an air conditioner (1st report,proposed reduced‐mesh analysis modeling method)

To analyze the performance of a heat exchanger (HEX) used in air conditioners within a realistic time frame, a simple method—based on a reduced‐mesh calculation model—was developed. The pressure loss caused by the HEX is given by a momentum source term, and the heat‐transfer performance of the HEX is approximated by using a wall function as the boundary condition. The analytical results from this simple analysis model (under a fin‐pitch range of 1.0 to 1.6 mm) were compared with experimental measurements of pressure loss and heat‐transfer performance. This comparison showed that the pressure loss from the simple analysis model agrees with the experimentally measured loss (within 3% error) and that the heating capacity determined by the simple model agrees with the experimentally measured one (within a 1% error). © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 12–23, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10130     

Study on forced convective heat transfer in curved pipes: (3rd report,theoretical analysis under the condition of uniform wall temperature and practical formulae)

In the authors' previous reports, the theoretical and experimental studies on flow and temperature fields in a curved pipe were made under the condition of uniform heat flux. In the former part of the present report, a theoretical analysis is made about temperature field far downstream from the pipe inlet under the condition of uniform wall temperature, following the same procedure as in the previous papers. Nusselt number is found to be remarkably affected by a secondary flow due to curvature. The result shows that in the first-order approximation the Nusselt number of heat transfer in a curved pipe does not differ for uniform wall temperature or uniform heat flux case, in both laminar and turbulent regions.In the latter part, the formulae of Nusselt numbers obtained by the authors' study are arranged so as to have simpler expression for a practical use.It is also investigated what temperature should be chosen in calculation of physical properties when these Nusselt number formulae are used.