台阶型旋转热管充液量的研究

建立了台阶型旋转热管最佳充液量的理论分析模型，探讨了旋转热管的转速，几何尺寸，工作温度及传输功率对最佳充液量的影响规律，可为工程应用中不同工况下台阶型旋转热管最佳充液量的确定提供理论参考依据，同时，由该模型对冷凝段的传热系数进行了预测，并将预测值与实验值进行了对比分析，图8参4。     

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.     

Update on Condensation Heat Transfer and Pressure Drop inside Minichannels

The present paper reviews published experimental work focusing on condensation flow regimes, heat transfer, and pressure drop in minichannels. New experimental data are available with high (R410A), medium (R134a), and low (R236ea) pressure refrigerants in minichannels of different cross-section geometries and with hydraulic diameters ranging from 0.4 to 3 mm. Because of the influence of flow regimes on heat transfer and pressure drop, a literature review is presented to discuss flow regimes transitions. The available experimental frictional pressure gradients and heat transfer coefficients are compared with semi-empirical and theoretical models developed for conventional channels and models specifically created for minichannels. Starting from the results of the comparison between experimental data and models, the paper will discuss and evaluate the opportunity for a new heat transfer model for condensation in minichannels; the new model attempts to take into account the effect of the entrainment rate of droplets from the liquid film.     

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.     

Condensation heat transfer and pressure drop characteristics of R-134a in an annular helical pipe

Condensation heat transfer and pressure drop data of R-134a in annular helical pipes is of significant importance to the effective design and reliable operation of helical pipe heat exchangers for refrigeration, air-conditioning, and many other applications. This paper presents the experimental investigation on condensation heat transfer and pressure drop characteristics of R-134a in an annular helical pipe. The average condensation heat transfer coefficients and pressure drops were experimentally determined for R-134a at three different saturated temperatures (35 °C, 40 °C, and 46 °C). The experimental results are compared with the data available in the literature for helical and straight pipes.     

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.     

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.     

A Numerical Study of Flow and Heat Transfer in Internally Finned Rotating Curved Pipes

In this article, the effects of internal fins on an incompressible viscous flow and heat transfer inside rotating curved pipes are numerically studied under the hydrodynamically and thermally fully developed conditions. The fins are assumed to have negligible thickness with the same conditions as the pipe walls. Two thermal boundaries including constant wall temperature and constant heat flux are considered at the pipe wall. First the accuracy of the numerical code written by a finite-volume method based on the SIMPLE algorithm is verified by the available data for the finless rotating curved pipes. Then, the numerical results for the internally finned rotating pipes are investigated in both positive and negative rotation numbers affecting remarkably on the flow and temperature field patterns. Also, the Dean number (K_LC) effects on the friction factor, Nusselt number, and other nondimensional parameters are studied in detail. Analyzing the numerical results by the Colburn factor, two optimum fin heights consisting of the four-fifth of the pipe radius at the lower Dean numbers and one-third of the pipe radius at higher Dean numbers are determined in the curved rotating pipe with six internal fins.     

一种新型微热管传热性能的实验研究

对一种新型的平板式微热管一零切角曲面微热管进行了实验研究。以热阻为基础,研究不同倾角、工质、充液比下微热管的热性能。为便于分析,将热管总热阻分解为4个部分：加热热阻、蒸发段热阻、冷凝段热阻和热沉热阻。通过实验得出如下结论：微热管总热阻的主要变化因素是冷凝段热阻和蒸发段热阻;与相应的无工质平板式换热器相比,实验件主要热阻变为热沉热阻．蒸发段和冷凝段热阻所占比例较低。根据不同的充液比和倾角。微热管传热极限分别由局部干烧和核态沸腾向膜态沸腾转化引起。实验表明。这种新型的微热管具有良好的应用前景,但是对于其机理还需要更深入的研究。     

Heat eddy diffusivity for viscoelastic turbulent pipe flows

A new semi-empirical equation for eddy diffusivity of heat in terms of friction drag reduction ratio and Weissenberg number was earlier reported. The assumptions made in the evaluation of the constants of the proposed equation are verified and its general applicability to various experimental data is further established. For this purpose experiments have been performed for Separan AP-273 and Polyox WSR-301 solutions with concentrations ranging from 10 to 1000 ppm and Separan AP-30 with concentration of 3000 ppm in thermally fully developed turbulent flow in pipes with diameters of 1.11 and 1.88 cm I.D. under constant wall heat flux. From the experiments it is concluded that the minimum asymptotes for friction and heat transfer and the critical Weissenberg number for heat transfer are universal and independent of the type of polymer used. The prediction of heat transfer coefficients with the use of the proposed equation for all of the experimental data is within a maximum deviation of 30%.     

Evaporation heat transfer and pressure drop of refrigerant R-134a in a small pipe

An experiment was carried out to investigate the characteristics of the evaporation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe having an inside diameter of 2.0 mm. The data are useful in designing more compact and effective evaporators for various refrigeration and air conditioning systems. The effects of the imposed wall heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured evaporation heat transfer and pressure drop were examined in detail. When compared with the data for larger pipes (D_i ≥ 8.0 mm) reported in the literature, the evaporation heat transfer coefficient for the small pipe considered here is about 30–80% higher for most situations. Moreover, we noted that in the small pipe the evaporation heat transfer coefficient is higher at a higher imposed wall heat flux except in the high vapor quality region, at a higher saturation temperature, and at a higher mass flux when the imposed heat flux is low. In addition, the measured pressure drop is higher for increases in the mass flux and imposed wall heat flux. Based on the present data, empirical correlations were proposed for the evaporation heat transfer coefficients and friction factors.     

Effects of turbulence model and interfacial shear on heat transfer in turbulent falling liquid films

An improved method is presented for the prediction of heat transfer coefficients in turbulent falling liquid films with or without interfacial shear for both heating or condensation. A modified Mudawwar and El-Masr's semi-empirical turbulence model, particularly to extend its use for the turbulent falling film with high interfacial shear, is used to replace the eddy viscosity model incorporated in the unified approach proposed by Yih and Liu. The liquid film thickness and asymptotic heat transfer coefficients against the film Reynolds number for wide range of interfacial shear predicted by both present and existing methods are compared with experimental data. The results show that, in general, predictions of the modified model agree more closely with experimental data than that of existing models.     

Study on overall heat transfer coefficient for a rotating cavity type heat exchanger

In this study, overall heat transfer coefficient for a rotating cavity type heat exchanger has been investigated. The heat exchanger mainly consisted of a rotating cylindrical cavity with axially throughflow. The cylindrical cavity was rotated in a stationary housing. An axial throughflow of cold water was supplied in the cavity through a pipe rotating with cavity. The cold water left the cavity via an identical pipe. Hot water flowed between rotating cylindrical cavity and stationary housing Experiments were conducted to obtain the effects of cold water flow rate, hot water flow rate, and rotational speed on overall heat transfer coefficient.     

Direct-contact condensation of pure steam on co-current and counter-current stratified liquid flow in a circular pipe

We carried out a set of experiments on the direct-contact condensation of atmospheric steam for subcooled water flowing co-currently and counter-currently in a circular pipe. The condensation heat transfer coefficient was evaluated both for co-current and counter-current steam–water flow cases in a horizontal circular pipe. In the current experiment the dependency of the liquid Nusselt number on the gas Reynolds number is higher in the counter-current than in the co-current experimental data. The dependency of the liquid Nusselt number on the steam Reynolds number is stronger in the rectangular channel than in the circular pipe. The overall heat transfer characteristics are better in the co-current flow than in the counter-current flow with the same injection flow rates of the steam and the water. The present co-current experimental data were used to assess four existing correlations. However, there are few reliable correlations existing to predict co-current experimental data. The comparisons of the present counter-current experimental data with the existing correlations show that Chu’s (Chu, I.C., Yu, S.O., Chun, M.H. 2000. Interfacial condensation heat transfer for counter-current steam–water stratified flow in a circular pipe, J. Korea Nucl. Soc., 32 (2), 142–156) correlation predicts the experimental data well.     

A New Numerical Approach for Predicting the Two-Phase Flow of Refrigerants during Evaporation and Condensation

This article reports on four finite-volume–based numerical methods developed for predicting the one-dimensional two-phase flow of pure refrigerants in evaporators and condensers during change-of-phase processes. The methods differ in the physical assumptions considered at the interface separating the liquid and vapor phases and in the equation used to predict the variation of the refrigerant flow quality during change of phase. In all methods, numerical predictions are obtained via a locally iterative marching-type solution algorithm. Therefore, the models permit the prediction of the size of the pipe needed to achieve full evaporation/condensation of the saturated refrigerant. The effectiveness and robustness of the numerical procedures in predicting the flow and heat transfer characteristics are assessed by comparing results with published experimental data. Good agreement is obtained. The new approach is used to perform a parametric study analyzing the effect of refrigerant type, pipe diameter, and mass flow rate on the flow and heat transfer characteristics in evaporators.     

A new heat eddy diffusivity equation for calculation of heat transfer to drag reducing turbulent pipe flows

A new semi-empirical equation of heat eddy diffusivity in terms of friction drag reduction ratio and Weissenburg number is presented. The proposed equation was validated with heat transfer experimental results of Kwack [1] and our recent experimental results, both for aqueous solutions of polyacrylamide (Separan AP-273) for concentrations ranging from 10 to 1000 ppm in turbulent flow through pipes under constant wall heat flux condition. The predictions of heat transfer coefficients with the use of the proposed equation are in good agreement with both sets of independent experimental results. The results of this study indicate that the proposed equation for eddy diffusivity of heat has predictive capability provided experimental measurements of pressure drop and the fluid time scale are available. The fluid time scale for these prediction was estimated using the Powell-Eyring fluid model and apparent viscosity measurements.     

Evaporation heat transfer of falling water film on a horizontal tube bundle

A numerical simulation and experimental study were carried out for evaporation heat transfer of a falling water film on a smooth horizontal tube bundle evaporator. A laminar model and a turbulence model were respectively adopted to calculate the heat transfer coefficients of falling water film on horizontal heated tubes. The calculation zone on the heated tube was divided into the top stagnation zone and the lateral free film zone. The initial boundary conditions for the free film zone were determined from the calculated results of the stagnation zone. The modified wall function method was used for the turbulent flow. Comparisons between the experimental data and the numerical solutions by use of two flow models show that the experimental data lie between the laminar model solutions and the latter turbulence model solutions and that they are closer to the latter solutions. Finally, a simple dimensionless correction based on the numerical simulations is proposed for predicting the evaporation heat transfer of falling water film for actual engineering applications. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 42–55, 2002     

Evaporation and condensation heat transfer in a heat pipe with a sintered-grooved composite wick

A mathematical model of evaporation and condensation heat transfer in a copper-water wicked heat pipe with a sintered-grooved composite wick is developed and compared with experiments. The wall temperatures are measured under different input power levels and working temperature conditions. The results show that the heat transfer in the condenser section was found to be only by conduction. In the evaporator, however, either conduction or boiling heat transfer can occur. The experimental data for the boiling heat transfer are well correlated by the theory of Stralen and Cole. Higher heat load drives the heat pipe to spend more time achieving the equilibrium state during the transient start-up process. The response curves of the evaporator thermal resistance are overlapped, and the condenser thermal resistance increases more sharply at the beginning. The total thermal resistance of the heat pipe ranges from 0.02 to 0.56 K/W.     

Thermal performance and operational attributes of the startup characteristics of flat-shaped heat pipes using nanofluids

Thermal performance, transient behavior and operational start-up characteristics of flat-shaped heat pipes using nanofluids are analyzed in this work. Three different primary nanofluids namely, CuO, Al₂O₃, and TiO₂ were utilized in our analysis. A comprehensive analytical model, which accounts in detail the heat transfer characteristics within the pipe wall and the wick within the condensation and evaporation sections, was utilized. The results illustrate enhancement in the heat pipe performance while achieving a reduction in the thermal resistance for both flat-plate and disk-shaped heat pipes throughout the transient process. It was shown that a higher concentration of nanoparticles increases the thermal performance of either the flat-plate or disk-shaped heat pipes. We have also established that for the same heat load a smaller size flat-shaped heat pipe can be utilized when using nanofluids.     

Heat transfer in the evaporator section of moderate-speed rotating heat pipes

Experiments were performed to investigate the heat transfer mechanism in the evaporator section of non-stepped rotating heat pipes at moderate rotational speeds of 2000–4000 rpm or accelerations of 40g–180g, and evaporator heat fluxes up to 100 kW/m². The thermal resistance of the evaporator section as well as that of the condenser section was examined by measuring the axial temperature distributions of the flow in the core region of the heat pipe and along the wall of the heat pipe. The experimental results indicated that natural convection heat transfer occurred in the liquid layer of the evaporator section under these conditions. The heat transfer measurements were in reasonable agreement with the predictions from an existing rotating heat pipe model that took into account the effect of natural convection in the evaporator section.