Heat transfer characteristics of the evaporator section using small helical coiled pipes in a looped heat pipe

An experimental study was carried out for the heat transfer characteristics and the flow patterns of the evaporator section using small diameter coiled pipes in a looped heat pipe (LHP). Two coiled pipes: the glass pipe and the stainless steel pipes were used as evaporator section in the LHP, respectively. Flow and heat transfer characteristics in the coiled tubes of the evaporator section were investigated under the different filling ratios and heat fluxes. The experimental results show that the combined effect of the evaporation of the thin liquid film, the disturbance caused by pulsation and the secondary flow enhanced greatly the heat transfer and the critical heat flux of the evaporator section. In final, two dimensionless empirical correlations were proposed for predicting the heat transfer coefficients of the evaporator section before and after dryout occurs.     

Heat transfer analysis of stratified flow in rotating heat pipes with cylindrical and stepped walls

This paper deals with the flow behavior and the related heat transfer characteristics of stratified flow in axially rotating heat pipes with cylindrical and stepped wall configurations. Flow patterns are presented with existing experimental data of heat transfer in cylindrical and stepped wall rotating heat pipes. Theoretical and semi-empirical models for calculation of the condensation and evaporation heat transfer coefficients are developed. Key dimensionless numbers such as Froude, Galileo, G and ξ-number are identified. Existing experimental data from a rotating cylindrical heat pipe are analyzed and used for regression based on semi-empirical models. Good agreement between the predicted results and experimental data was obtained. Comparison between the present heat transfer models rotating cylindrical wall heat pipes and experimental data from a stepped wall heat pipe shows that the present models can be used to predict the condensation and evaporation heat transfer coefficients in a rotating stepped wall heat pipe.     

Experimental investigation on the heat transfer characteristics of axial rotating heat pipes

The heat transfer performance of axial rotating heat pipes was measured under steady state at rotational speeds up to 4000 RPM, or a maximum centrifugal acceleration of 170g, and heat transfer rates up to 0.7 kW. A cylindrical and an internally tapered heat pipe with water as the working fluid were tested with different fluid loadings that ranged from 5% to 30% of the total interior volume. The measurements were used to characterize the effects of rotational speed, working fluid loading, and heat pipe geometry on the heat transfer performance. The internal taper on the condenser was found to significantly increase the heat transfer rate compared to the cylindrical case. A comparison between the test results and predictions from previous models showed that natural convection in the liquid film at the heat pipe evaporator plays an important role in the heat transfer mechanism at high rotational speeds.     

Fluid flow and heat transfer model for high-speed rotating heat pipes

A new complete model has been developed to predict the performance of high-speed rotating heat pipes with centrifugal accelerations up to 10 000 g. The flow and heat transfer in the condenser is modeled using a conventional modified Nusselt film condensation approach. The heat transfer in the evaporator has previously been modeled using a modified Nusselt film evaporation approach. It was found, however, that natural convection in the liquid film becomes more significant at higher accelerations and larger fluid loadings. A simplified evaporation model including the mixed convection is developed and coupled with the film condensation model. The predictions of the model are in reasonable agreement with existing experimental data. The effects of working fluid loading, rotational speed, and pipe geometry on the heat pipe performance are reported here.     

Theoretical and numerical analysis of convective heat transfer in the rotating helical pipes

In terms of the tensor analysis technique, the relative N-S equations and the energy equation in a rotating helical coordinate system are presented in this paper. Convective heat transfer in the rotating helical pipes with circular cross-section is investigated employing theoretical and numerical method. A perturbation solution up to the secondary order is obtained for a small Dean number. Variations of the temperature distribution with the force ratio (the ratio of the Coriolis force to the centrifugal force), the curvature and the torsion are discussed in detail. Present studies also show the natures of the Nusselt number, as well as the effects of the force ratio, the curvature, and the torsion. This study explores many new characteristics of convective heat transfer in the rotating helical pipes and covers wide ranges of parameters.     

Analogy of forced convective heat transfer between developing laminar flows in curved pipes and orthogonally rotating pipes

Based on a quantitative analogy between developing laminar flows in curved pipes and orthogonally rotating straight pipes, a corresponding analogy of forced convective heat transfer in the entry regions of the pipes is described through similarity arguments and computational studies. Three‐dimensional developments of these flows are characterized by secondary flows due to the centrifugal or the Coriolis forces. Similarity considerations taking the secondary flow into account suggest a remarkable effect of the Prandtl number or the heat transfer structure, which is demonstrated by the computational results. When the curvature parameter of curved pipe flow and the Rossby number of rotating pipe flow are sufficiently large, it is shown that the development of the temperature fields and the Nusselt numbers of the two flows are similar when the governing parameters and the Prandtl numbers of the two flows are equal. © 2000 Scripta Technica, Heat Trans Asian Res, 29(6): 512–522, 2000     

Experimental studies of rotating heat pipes

Thermal characteristics of a rotating heat pipe were measured under steady state at moderate rotational speeds. Copper‐water rotating heat pipe with copper screen mesh wick was fabricated for testing at various heat loads. An experimental test rig with a water‐cooled condenser section was fabricated to study the heat transfer in the rotating heat pipe (RHP) for various heat loads and various rotational speeds ranging from 1000 rpm to 2000 rpm. A heat transfer correlation was developed for the condensing heat transfer coefficient and compared with the experimental results. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20265     

A numerical study of flow and heat transfer in internally finned rotating straight pipes and stationary curved pipes

In this article the effects of internal fins on laminar incompressible fluid flow and heat transfer inside rotating straight pipes and stationary curved pipes are numerically studied under hydrodynamically and thermally fully developed conditions. The fins are assumed to have negligible thickness with the same conditions as the pipe walls. Two cases, constant wall temperature and constant heat flux at the wall, are considered. First the accuracy of the numerical code written by a finite volume method based on SIMPLE algorithm is verified by the available data for the finless rotating straight pipes and stationary curved pipes, and then, the numerical results for those internally finned pipes are investigated in detail. The numerical results for different sizes and numbers of internal fins indicate that the flow and temperature field analogy between internally finned rotating straight pipes and stationary curved pipes still prevail. The effects of Dean number (K_L) versus friction factor, Nusselt number, and other non-dimensional parameters are studied in detail. From the numerical results obtained, an optimum fin height about 0.8 of pipe radius is determined for Dean numbers less than 100. At this optimum value, the heat transfer enhancement is maximum, and the heat transfer coefficient appears to be 6 times as that of corresponding finless pipes.     

Wall-particles heat transfer in rotating heat exchangers

The wall-particles heat transfer coefficient has been measured in small-scale rotary drum heat exchangers. Experiments have been conducted with nine granular materials of different nature, with particle diameters ranging from 194 μm to 4mm. The effects of rotational speed (1–40 rev min⁻¹), filling degree (4–17%) and drum diameter (0.25 and 0.485 m) have been investigated. The experimental data have been correlated by a semi-empirical relationship, that includes a contact resistance at the wall, the heat capacity of the particles immediately adjacent to the wall and the heat penetration resistance of the bulk of the particle bed. The contact resistance is shown to be due to the roughness of the particles. A mean roughness height of 12 μm provides a good fit to the measured coefficients.     

Heat and mass transfer of ammonia-water in falling film evaporator

To investigate the performance of heat and mass transfer of ammonia-water during the process of falling film evaporation in vertical tube evaporator, a mathematical model of evaporation process was presented, the solution of which that needed a coordinate transformation was based on stream function. The computational results from the mathematical model were validated with experimental data. Subsequently, a series of parameters, such as velocity, film thickness and concentration, etc., were obtained from the mathematical model. Calculated results show that the average velocity and the film thickness change dramatically at the entrance region when x<100 mm, while they vary slightly with the tube length in the fully developed region when x>100 mm. The average concentration of the solution reduces along the tube length because of evaporation, but the reducing tendency becomes slow. It can be concluded that there is an optimalβrelationship between the tube length and the electricity generated. The reason for the bigger concentration gradient in the y direction is that the smooth tube is chosen in the calculation. It is suggested that the roll-worked enhanced tube or other enhanced tube can reduce the concentration gradient in the film thickness direction and enhance the heat and mass transfer rate.     

Comparative study on evaporator heat transfer characteristics of revolving heat pipes filled with R134a, R22 and R410A

Heat pipes are used extensively in various applications including the heating, ventilating and air conditioning (HVAC) systems. The high thermal conductivity of the device, attributed from the two-phase heat transfer processes within the heat pipe, made them superior heat exchanger devices. Heat pipes had been widely used in HVAC applications in energy conservation, dehumidification enhancement, heat dissipation, etc. A number of researches have been conducted to expand the applicability of heat pipes in HVAC in Malaysia, especially in air-to-air heat recovery using stationary heat pipes. However, the potential usage of rotating heat pipe in heat recovery in tropical countries like Malaysia was yet to be explored. Hence, the potential of rotating heat pipe in the HVAC systems used in tropics was explored through a parametric study that incorporates rotational speeds, off-axis displacements and varied refrigerants. The rotating heat pipes charged with R134a, R22 and R410A were tested with varied radial displacement from the rotational axis. The straight and leveled heat pipe with the furthest radial displacement yields the most significant heat transfer, which was attributed to the magnitude of the generated centrifugal force, and effective distribution of liquid in the evaporator.     

Visualization and thermal resistance measurement for the sintered mesh-wick evaporator in operating flat-plate heat pipes

This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26 mm to 0.8 mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1 × 1.1 cm². At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–100 W/cm². Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dryout appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance re-rose in response to the appearance and expansion of partial dryout. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances.     

Heat transfer from rotating annular fins

Experiments have been performed to determine the heat transfer coefficients for arrays of shaft-attached, rotating annular fins. The experiments encompassed a wide range of rotational speeds and interfin spacings (including the limiting case of the single annular fin). The efficiency of the fins was equal to one. It was found that the fin heat transfer coefficient decreased with decreasing interfin spacing, the extent of the decrease being of major proportions at low rotational speeds but being quite moderate at high speeds. Thus, closely spaced fins can be used at high rotational speeds without a significant spacing-related decrease in the transfer coefficient, but at low speeds the fins must be farther apart to avoid overly low values of the coefficient. The heat transfer coefficient also decreased as the rotational speed decreased, with a particularly rapid dropoff at low speeds when the interfin spacing was small. For the most part, the fin heat transfer coefficients substantially exceeded those for an unfinned rotating shaft, thereby providing an incentive for finning. It was also found that at high rotational speeds, the heat transfer coefficient for a rotating disk served as a lower bound for the annular-fin heat transfer coefficients. To facilitate the use of the results for design, a correlation was developed which represents the fin heat transfer coefficient as a continuous function of the investigated independent parameters.     

Evaporation resistance measurement with visualization for sintered copper-powder evaporator in operating flat-plate heat pipes

The evaporation resistances of loosely-sintered copper-powder evaporators were measured in operating flat-plate heat pipes. The evaporation processes was also visualized through a top glass plate. Irregular or spherical powders of different size distributions were investigated. Uniform heating of 16–170 W/cm² was applied to the base plate near one end with a heated surface of 1.1 × 1.1 cm². At the other end was a cooling water jacket. The evaporation performance was first examined with the effect of liquid flow resistance minimized, i.e., the copper powders covered only the heated area with the remaining region covered with sintered copper wire screens. Similar to multi-layer mesh wicks, quiescent surface evaporation without nucleate boiling was observed for all test conditions, in spite of the abundant nucleation sites. The water film receded and the evaporation resistance reduced with increasing heat flux. Once partial dryout occurred, the evaporation resistance re-rose. The minimum evaporation resistances were about 0.08–0.09 W cm²/K for wicks containing fine powders. These values are similar with those for multi-layer-mesh wicks having a fine bottom screen. In the absence of fine powders, the minimum evaporation resistances were significantly larger. In the second part for homogeneous sintered-powder wicks, the large flow resistance tended to retard the condensed water from returning to the evaporator. However, this can be compensated by a larger charge and/or a thicker wick.     

Heat transfer behavior in a rotating aluminum foam heat sink with a circular impinging jet

This work experimentally studied heat transfer associated with an impinging jet onto a rotating heat sink. Air was used as the impinging coolant, and a square Al-foam heat sink was adopted. The variable parameters were the jet Reynolds number (Re), the relative nozzle-to-foam tip distance (C/d), the rotational Reynolds number (Re_r) and the relative side length of the square heat sink (L/d). The effects of Re, C/d, Re_r and L/d on the dimensionless temperature distributions and the average Nusselt number were considered. For a stationary system, the results reveal that the average Nusselt number (Nu₀) with Al-foam was two to three times that without Al-foam. Nu₀ increased with Re. A larger L/d responded to a larger Nu₀ based on the same jet flow rate. The effect of C/d on Nu₀ was negligible herein. For a rotating system, when Re and L/d were small and C/d was large, the average Nusselt number (Nu_Ω) increased considerably with Re_r. Additionally, for Nu_Ω/Nu₀ ? 1.1, the results suggest that rotation was substantial at Re_r/Re ? 1.13 when L/d = 4.615 with C/d = 0–5 and at Re_r/Re ? 1.07 when L/d = 3.0 with C/d = 0–5. For L/d = 2.222, rotation was substantial at Re_r/Re ? 1.44 when C/d = 0 and was always substantial when C/d ? 1.     

LHP圆盘式蒸发器三维传热与流动分析

通过补充相变蒸发界面传热传质热力学关系式,构建了回路热管(loop heat pipe,LHP)圆盘式蒸发器的流动与传热多区域耦合分析三维数学物理模型,并基于FLUENT软件对某种甲醇-不锈钢平板型蒸发器内的流动与传热情况进行了数值求解。数值分析结果表明,蒸发器内的传热与流动受几何结构影响明显,表现出较强的方向差异;不同热负荷条件下,补偿腔内流体的流动与传热特性呈现出较大差别,受到回流液速度和温度、毛细芯界面蒸发质量流量、毛细芯反向导热和侧壁漏热等多种因素共同影响。计算方法和研究结果,可以为平板型蒸发器内流动与传热特性的定量分析提供依据。     

Effect of interfacial phenomena on evaporative heat transfer in micro heat pipes

A three-dimensional steady-state model for predicting heat transfer in a micro heat pipe array is presented. Three coupled models, solving the microregion equations, the two-dimensional wall heat conduction problem and the longitudinal capillary two-phase flow have been developed. The results, presented for an aluminium/ammonia triangular micro heat pipe array, show that the major part of the total heat input in the evaporator section goes through the microregion. In addition, both the apparent contact angle and the heat transfer rate in the microregion increase with an increasing wall superheat. It is also shown that the inner wall heat flux and temperatures as well as the contact angle decrease all along the evaporator section.     

平板蒸发器环路热管启动及充灌率特性研究

结合环路热管系统的优点与太阳能的广泛利用前景,设计并搭建了以太阳能利用为背景的新型毛细芯平板蒸发器环路热管系统,采用泡沫镍为毛细芯、乙醇为工质,实验研究了系统的启动运行特性,以及不同的工质充灌率对环路热管系统性能的影响。结果显示,在实验条件下,环路热管热源功率在300~1 600 W时具有良好的启动运行特性,55%的充灌率为最佳充灌率,具有更短的启动时间,相对更低的蒸发器温度与热阻。     

Heat transfer and exergy analysis of cascaded latent heat storage with gravity-assisted heat pipes for concentrating solar power applications

A thermal network model is developed to predict the performance of latent heat thermal energy storage (LHTES) systems including cascaded phase change materials (PCMs) and embedded heat pipes/thermosyphons. Because the design of LHTES systems involves a compromise between the amount of energy stored, the heat transfer rate, and the quality of the released thermal energy, an exergy analysis is also carried out to identify the preferred LHTES design. It is found that the LHTES with the lowest melting temperature PCM yields the highest exergy efficiency. However, a cascaded LHTES recovers the largest amount of exergy during a 24 h charging–discharging cycle. Quantitatively, the cascaded LHTES recovers about 10% more exergy during a 24 h charging–discharging cycle compared to the best non-cascaded LHTES considered in this work.     

Upflow turbulent mixed convection heat transfer in vertical pipes

The present work deals with the results of an experimental investigation on heat transfer in water cooled vertical pipes, for thermal–hydraulic conditions ranging from forced convective flow to mixed convective flow. The flow of water in the pipe is upwards.Experimental data confirm the reduction in the heat transfer rate for mixed convection in upward heat flow, mainly due to the laminarization effect in the near-wall region (buoyancy effect) . They are in a very good agreement with numerical methods, such as the k–-model.A new method for the calculation of the heat transfer coefficient in upward mixed convection heated flow is proposed. It is based on the well-known superposition method (heated downflow) modified accounting for the phenomenology of the upward heated flow in comparison with downflow heated conditions.