热分层弯管热波动及其削弱机理研究

热分层弯管中的热波动容易诱发热疲劳,越来越受到核电工程与技术的关注。对弯管内热分层的流动与传热机理进行研究,建立数学模型,运用FLUENT软件,对有/无防涡器条件下T型弯管内流体的温度波动过程进行了大涡模拟,获得了弯管处无量纲温度波动曲线和无量纲均方根温度。数值模拟结果表明,加入防涡器后,由于管内湍流减弱,防涡器下方弯管处温度波动减弱,热波动受到抑制。     

Flashing characteristics in a pipe downstream from a depressurizing tank and temperature fluctuation characteristics at a mixing tee junction with cold water injection

The flashing characteristics in a pipe downstream from a depressurizing tank were experimentally and analytically investigated on the basis of the transient test and two‐phase flow analysis. The following conclusions were obtained. (1) When the pressure margin of the pump inlet side and the distance to obtain an isothermal condition were sufficient, flashing phenomena did not occur in spite of the decreasing pressure. (2) When the ratio of the cold water injection flow rate to the hot water flow rate M_c/M_h increased, the peak distance of the water temperature fluctuation moved from L/D = 1 to 0, and the maximum water temperature fluctuation ratio was about 40% of the temperature difference between hot and cold water near the mixing tee junction. Because no problem occurred regarding the pipe material thermal fatigue, reliability of the mixing tee junction was assured. (3) Due to suppression of flashing phenomena of the mixing pipe system, the decision diagram on the flashing occurrence was obtained from the test and the analytical results, taking into consideration three factors: the depressurizing ratio in the tank; the cold water injection flow rate due to remaining subcooling; and the delay time of thermal mixing. The simplified analytical equation was used to decrease the cold water injection flow rate by the optimized pipe length between the mixing tee junction and the drain pump. The cold water injection flow rate was minimized when the pipe length was about 15 to 20 times the pipe inner diameter. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 411–429, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10096     

Modeling of hot gas flow through a feed stream within a horizontal pipe

High heat penetration into a feed stream within a horizontal pipe is described mathematically with a gas flow and heat transfer model. Influences of varied factors on the gas flow and heat transfer in porous media are examined for different conditions. The temperature of the packed‐bed particles and the gas velocity distribution curves are obtained for the feeding service at interruption and at regular charge operating conditions. The numerical results show that the thermal effect to the packed‐bed particles by the seepage flow fluid is high only in the position near the gas entrance. The thermal penetration depth tends to increase with the seepage flow velocity and decrease with the feed rate. The operating conditions and the porosity of solid bed have importance effects on the gas velocity and temperature field in the thermal penetration zone. The model results are found to compare favorably with the experimental data available in the literature. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(6): 553–565, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10109     

Natural convective heat transfer in uniformly heated vertical pipe annuli

Natural convective heat transfer in vertical concentric pipe annuli is investigated both numerically and experimentally for a fluid having a Prandtl number of 0.7. Numerical calculations for three cases of different heating conditions for pipes (heated inner pipe, heated outer pipe, both pipes heated) are made of laminar flows for different inner‐to‐outer‐pipe diameter ratios d_i/d_o from 0.2 to 0.8. For each case, the thermal entrance length x/b at the modified Grashof numbers Gr^*=10² to 5 × 10⁵ is well correlated with Grashof number Gr^* and annulus length to clearance ratio L/b. Local Nusselt numbers Nu_i and Nu_o in the thermally fully developed region have certain constant values dependent on the diameter ratio d_i/d_o, regardless of Gr^* and L/b. Average Nusselt numbers Nu_i and Nu_o in the thermal entrance region are also independent of Gr^* and L/b. © 2001 Scripta Technica, Heat Trans Asian Res, 30(8): 676–688, 2001     

Theoretical and experimental investigation of a heat pipe heat exchanger for energy recovery of exhaust air

Heat pipes and two-phase thermosyphon systems are passive heat transfer systems that employ a two-phase cycle of a working fluid within a completely sealed system. Consequently, heat exchangers based on heat pipes have low thermal resistance and high effective thermal conductivity, which can reach up to the order of (10⁵ W/(m K)). In energy recovery systems where the two streams should be unmixed, such as air-conditioning systems of biological laboratories and operating rooms in hospitals, heat pipe heat exchangers (HPHEs) are recommended. In this study, an experimental and theoretical study was carried out on the thermal performance of an air-to-air HPHE filled with two refrigerants as working fluids, R22 and R407c. The heat pipe heat exchanger used was composed of two rows of copper heat pipes in a staggered manner, with 11 pipes per row. Tests were conducted at different airflow rates of 0.14, 0.18, and 0.22 m³/h, evaporator inlet-air temperatures of 40, 44, and 50°C, filling ratios of 45%, 70%, and 100%, and ratios of heat capacity rate of the evaporator to condenser sections (C_e/C_c) of 1 and 1.5. For HPHE's steady-state operation, a mathematical model for heat-transfer performance was set and solved using MATLAB. Results illustrated that the heat transfer rate was in direct proportion with the evaporator inlet-air temperature and flow rate. The highest HPHE's effectiveness was obtained at a 100% filling ratio and (C_e/C_c) of 1.5. The predicted and experimental values of condenser outlet-air temperature were in good agreement, with a maximum difference of 3%. HPHE's effectiveness was found to increase with the increase in evaporator inlet-air temperature and number of transfer units (NTU) and with the decrease in airflow rate, up to 33% and 20% for refrigerants R22 and R407c, respectively. Refrigerant R22 was the superior of the two refrigerants investigated.     

Characterization of internal thermohydraulic flow and heat transfer improvement in a three-dimensional circular corrugated tube surfaces based on numerical simulation and design of experiment

To meet the requirements of development in heat exchangers design, the effect of different tubes geometrical parameters on its flow field analysis and thermal heat transfer performance are investigated in the current research work. The hydraulic thermal fluid coupling with computational simulations is applied. The numerical results are solving used flow transport and heat transfer equations, then these results are validated with available experimental data. The behavior of hydraulic and thermal flow in the corrugated tube is discussed with different geometrical parameters' position and shape. Turbulent flow in the tube is calculated in three-dimensional numerical simulations with optimization of a multiobjective algorithm are analyzed. The influences of various design parameters, for instance, the number of corrugated rings around the tube, distance between each corrugated ring, the diameter of the ring, and pitch of ring are investigated firstly in the flow field and then optimized by using the design of experiment (DOE). The influence of flow structural modifications such as static pressure, dynamic pressure, and pressure drop is taken into consideration as analyzed performance parameters. The DOE method is investigated based on implements and variances the L16 orthogonal arrays are chosen as the experimental strategy. Furthermore, the optimization results found that the maximum value of pressure difference was for corrugated diameter. The numerical method using DOE has enhanced heat transfer rate as compared to the smooth pipe. Moreover, the minimum T_out is for Case 11 (296.49°C) and the maximum T_out is for (303.10°C) hence the value of Nu number for both cases is 32.9 and 42, respectively. That means using this type of passive device can improve the heat transfer in the pipe. The outcomes illustrate that the performance evaluation factor (PEF) ratio of the corrugated pipe with different geometrical configurations is changed and increased as the corrugated pipe geometrically changed and the value of PEF is more than 1.3.     

Experimental and numerical study on charging processes of an ice‐on‐coil thermal energy storage system

In this study, an external melt ice‐on‐coil thermal storage was studied and tested over various inlet conditions of secondary fluid—glycol solution—flow rate and temperature in charging process. Experiments were conducted to investigate the effect of inlet conditions of secondary fluid and validate the numerical model predictions on ice‐on‐coil thermal energy storage system. The total thermal storage energy and the heat transfer rate in the system were investigated in the range of 10 l min ^?1?V??60 l min ^?1. A new numerical model based on temperature transforming method for phase change material (PCM) described by Faghri was developed to solve the problem of the system consisting of governing equations for the heat transfer fluid, pipe wall and PCM. Numerical simulations were performed to investigate the effect of working conditions of secondary fluid and these were compared with the experimental results. The numerical results verified with experimental investigation show that the stored energy rises with increasing flow rate a decreasing tendency. It is also observed that the inlet temperature of the fluid has more influence on energy storage quantity than flow rate. Copyright © 2006 John Wiley & Sons, Ltd.     

多孔介质中高温气体非稳态渗流传热数值计算

针对水平导管中填充颗粒物料层内的高温气体参流传热现象，考虑渗流与传热的相互作用并采用局部非平衡假设建立多孔介质中的瞬态渗流传热物理数学模型。研究不同情况下填充物料中的渗流速度和气固温度分布。计算结果表明，高温热气体对水平导管中移动颗粒料层的热渗透主要发生在渗流入口端区域，随着渗流时间延长，热渗透深度沿导管推进。增大入口渗流速度以及减小出料速度，将导致物料温度沿导管慢速下降，热渗透深度扩大，热渗透作用区域内的物料温度水平提高。在热渗透作用区域，孔隙率对流场和温度场有很大的影响。研究对于高温反应器的颗粒输运和给料器的设计与运行有一定的参考作用。     

A numerical study to investigate the effect of turbulators on thermal flow and heat performance of a 3D pipe

To reduce the heat exchanger's costs in a highly competitive industry, thermal performance enhancement of the heat exchangers has successfully gained attention in the last few decades. Among different engineering approaches, the application of the enhanced pipes provides a key solution to improve heat performance. In this paper, the investigation develops a numerical study based on the commercially available computational fluid dynamics codes on the turbulent flow in three-dimensional tubular pipes. Various concavity (dimple) diameters with corrugation and twisted tape configurations are investigated. The study has shown that perforated geometrical parameters lead to a high fluid mixing and flow perturbation between the pipe core region and the walls, hence better thermal efficiency. Moreover, a model of concavity (dimple) with a 4 mm diameter allows the highest heat transfer enhancement among other designs. In addition, the study shows that due to the disturbance between the pipe core region and the pipe wall, the transverse vortices and swirl flow generated are forceful, which leads to better heat transfer enhancement compared with the conventional (smooth) pipes. As the Reynolds number (Re) rises, the mixing flow, secondary, and separation flow extend to become higher than the values in a smooth pipe, allowing a higher value of performance evaluation factor to be achieved for a dimple diameter of 1mm at the low Re values. This study, therefore, shows the promising potential of the enhanced pipes in the heat transfer enhancement of heat exchangers that is crucial in industrial applications to save more energy.     

Simulation of mixed convection heat transfer in a horizontal channel with an open cavity containing a heated hollow cylinder

The objective of this paper is to numerically investigate the mixed convective flow and heat transfer controlled by a heated hollow cylinder inside an open cavity attached with a horizontal channel. All the boundaries of the channel and cavity are perfectly insulated while the inner surface of the cylinder is heated uniformly by heat flux q. The equations of conservation of mass, momentum, and energy were solved using adequate boundary conditions by Galarkin's weighted residual finite element technique. The solution has been performed in the computational domain as a whole with proper treatment at the solid/fluid interface. Computations have been conducted for Ra = 10³–10⁵, Prandtl number Pr varying from 0.7 to 7 and ratio of solid to fluid thermal conductivities from 0.2 to 50. Results are presented in terms of streamlines, isotherms, heat transfer rate in terms of the average Nusselt number (Nu_av), drag force (D), and maximum bulk temperature (θ_max). © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21002     

Calibrated models for simulation of stratified hot water heat stores

Hot water heat stores (HWHS) are generally used to overcome the diurnal or seasonal mismatch in the availability and demand of thermal energy. To enhance the system efficiency, good thermal stratification of the HWHS is required. In order to simulate different flow processes in stratified HWHS the effects of stratification on the turbulence are to be considered. Benchmark experiments have been conducted on turbulent flows into a continuously stratified HWHS. Based on these benchmark experiments, different two‐equation turbulence transport models namely the RNG (ReNormalizable Group) and the realizable k–ε turbulence models have been calibrated. The major improvement is provided to the ε‐equation by introducing the effects of the buoyancy field on the turbulence dissipation rate. It is achieved by calibrating the coefficient of the dissipation term (C_ε2 in the RNG and C₂ in the realizable k–ε model) based on the benchmark experiments. A re‐definition of the turbulent Prandtl number (Pr_t) incorporating the effects of stratification on turbulent thermal diffusivity improved the calibration further. The calibrated computational fluid dynamic models are found to predict the charging, discharging and storing processes of typical HWHS with good accuracy. Copyright © 2008 John Wiley & Sons, Ltd.     

Rules‐of‐thumb of implementing short electric band heaters (length to diameter ratio <1.5) for external heating of pipe flows

Short electric band heaters (L/D_i<1.5) are constructed for the ease of implementation in small scale heating applications. They are usually mounted side‐by‐side in series along the external wall of a pipe for heating the fluid within the pipe. There are no rules‐of‐thumb available about designing such a system to achieve good uniformity of the temperature profile at the pipe inner surface beforehand. Non‐uniformity can cause preferential fouling at hotter spots. This study focuses on the axial uniformity of heating along a pipe inside which the heated fluid if flowing. The situation has been simplified a great deal in mathematical terms from the corresponding conventional conjugate problem considered previously due to the small temperature rise in the fluid flow through one section of the pipe which is heated by one band heater. Similarity parameter sets have been deduced and verified by numerical simulations. The worst scenario of non‐uniformity for such short band heaters, that is when L/D_i=1.5, is presented in this paper. This may be used for designing a system to minimize the non‐uniformity in terms of choosing the right pipe material, percentage of heater wire coverage in the band heater, etc. Copyright © 2002 John Wiley & Sons, Ltd.     

Inverse design methodology to optimize sensible thermal energy storage systems working as rectifiers

The main goal of this paper is to present a methodology to achieve the optimal design of a sensible thermal energy storage system (T3S) working as a thermal rectifier. The system was composed by the heat storage material (HSM), distributed in a set of flat parallel plates, and the working fluid, both modeled by a simplified lumped element model (LEM). The ratio of operational outlet temperature range to source temperature oscillation is defined as the designed rectifying duty. Optimization procedure combines a one‐factor‐at‐a‐time (OFAT) and line search strategies in order to find optimal T3S design that satisfies the designed rectifying duty with the minimum HSM mass. The inverse design philosophy is applied to the optimal results to generalize the T3S dynamic behavior as functions fitting curves of the number of transfer unit (NTU) and the time constant τ . These fitting curves can be used to identify T3S geometric parameters, HSM thermal properties, fluid inlet conditions, among others, which guarantee the outlet fluid temperature to be found within the operational range with the minimum HSM mass. A three step‐by‐step sequence design methodology is presented and detailed, based on design charts from the NTU and τ correlations. The proposed design methodology is able to find the optimal plate length L, plate thickness e_s , and plate distance e_f that satisfies the designed rectifying duty for three test cases. These optimal T3S designs are simulated in a computer fluid dynamics (CFD) code, with deviations below 1.5% between the designed rectifying duty and the one simulated. With the proposed approach, several design solutions or configurations can be found for T3S operating as a thermal rectifier based on NTU and τ fitting curves submitted to a sinusoidal cyclic temperature input and with constant and uniform HSM and fluid properties.     

Experimental investigation of start-up and transient thermal performance of pumped two-phase battery thermal management system

In this study, a pumped two-phase battery thermal management system was developed, and its start-up and transient thermal performances were experimentally evaluated. The start-up behavior was characterized, and the effects of the flow rate, heat flux, and cold-source temperature on the start-up and transient thermal performances were examined. Three start-up modes were observed: fluctuating growth, temperature overshoot, and smooth growth. The fluctuating growth start-up mode appears to be suitable for battery cooling. The transient performance was improved when the flow rate was decreased, which resulted in a quicker start-up and lower average temperature (t_avg) and maximum temperature difference (∆t_max). Reducing the flow rate from 0.99 to 0.20 L/min significantly shortened the start-up time, lowered t_avg and ∆t_max, and increased the heat transfer coefficient (α) when the steady state was reached. Increasing the heat flux initially improved and then weakened the transient performance of the pumped two-phase system. Increasing the heat flux from 1.1 to 2.8 W/cm² initially reduced the start-up time and t_avg to 350 seconds and 1.5°C, respectively, but they then significantly increased to 360 seconds and 13.5°C, respectively. The transient t_avg and ∆t_max decreased with the cold-source temperature (t_cs), while the start-up time was independent of changes in t_cs.     

Simulation of compressible flow in high pressure buried gas pipelines

The aim of this work is to analyze the gas flow in high pressure buried pipelines subjected to wall friction and heat transfer. The governing equations for one-dimensional compressible pipe flow are derived and solved numerically. The effects of friction, heat transfer from the wall and inlet temperature on various parameters such as pressure, temperature, Mach number and mass flow rate of the gas are investigated. The numerical scheme and numerical solution was confirmed by some previous numerical studies and available experimental data. The results show that the rate of heat transfer has not a considerable effect on inflow Mach number, but it can reduce the choking length in larger f_DL/D values. The temperature loss will also increase in this case, if smaller pressure drop is desired along the pipe. The results also indicate that for f_DL/D = 150, decreasing the rate of heat transfer from the pipe wall, indicated here by Biot number from 100 to 0.001, will cause an increase of about 7% in the rate of mass flow carried by the pipeline, while for f_DL/D = 50, the change in the rate of mass flow has not a considerable effect. Furthermore, the mass flow rate of choked flow could be increased if the gas flow is cooled before entrance to the pipe.     

NATURAL CONVECTION IN A TWO-DIMENSIONAL DIFFERENTIALLY HEATED SQUARE ENCLOSURE UNDERGOING ROTATION

A numerical study of two-dimensional natural convection in a rotating and differentially heated square enclosure has been presented by solving the conservation equations of mass, momentum, and energy in a rotating coordinate system using the finite difference method. Considering air to be the fluid medium in the cavity, the results are presented for a wide range of Rayleigh numbers (Ra), Taylor numbers (Ta), and rotational Rayleigh numbers (Ra _w). It is found that a significant enhancement in heat transfer can be achieved due to rotational effects. At a particular Ra, increase in Ta results in an increase in frequency of oscillations of the dynamical variables and also gives rise to formation of a mushroom-shaped plume in the core of the cavity. For constant Ta, an increase in Ra results in formation of thinner thermal boundary layers at the isothermal walls and stable thermal stratification in the core of the cavity. The stratification becomes unstable when the Ta     

Ground water level influence on thermal response test in Adana,Turkey

For optimum design of borehole thermal energy storage (BTES) and ground sources heat pump (GSHP) applications, determination of underground thermal properties is required. The design and economic feasibility (number and depth of boreholes) of these systems need thermal conductivity of geological structure, λ (W m^?1 K^?1), and thermal resistance of ground heat exchanger, R (K W^?1 m). Thermal properties measured in laboratory experiments do not coincide with data of in situ conditions. Therefore, in situ thermal response test equipment has been developed and used in Canada, England, Germany, Norway, U.K., U.S.A. and Sweden to ensure precise designing of BTES systems. This paper describes the results and evaluations of the Adana continual thermal response test measurements. Copyright © 2007 John Wiley & Sons, Ltd.     

Finite time thermal analysis of ground integrated‐collector‐storage solar water heater with transparent insulation cover

A transparent honeycomb insulated ground integrated‐collector‐storage system has been investigated for the engineering design and solar thermal performance. The system consists of a network of pipes embedded in a concrete slab whose surface is blackened and covered with transparent insulation materials (TIM) and the bottom is insulated by the ground. Heat may be retrieved by the flow of fluid through the pipe. A simulation model has been developed; it involves the solution of the two‐dimensional transient heat conduction equation using an explicit finite‐difference scheme. Computational results have been used to determine the effect of such governing parameters as depth as well as pitch of the pipe network and collector material on the thermal performance of the system. The pipe network depth of 10 cm and the TIM cover made of 5 cm compounded honeycomb seem suitable for the proposed system. Solar gain (solar collection efficiency of 30–50% corresponding to collection temperature of 40–60°C) and the diurnal heat storage characteristics of the system are found to be of the right order of magnitude for solar water heating applications. Copyright © 1999 John Wiley & Sons, Ltd.     

Forced Convective Heat Transfer in a Plate Channel Filled with Solid Particles

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Obstacle's effects and their location inside the square cavity on the thermal performance of Cu–Al2O3/H2O hybrid nanofluid

The current study focuses on the effect of obstacles and their positioning within the square cavity (L = H) on heat exchange. This work considers heating the cavity's bottom wall to a steady, high temperature. The top wall of the cavity is adiabatic, while the two vertical side walls are cooled. Four cases are explored under these conditions: the first case is a square-shaped cavity holding a square-shaped obstacle h = l = 0,15 L, while the other three cases, respectively, each include two, three, and four square obstacles. The cavity was filled with Cu–Al₂O₃/H₂O hybrid nanofluid with a volume fraction φ = 0.03. Numerical results for laminar and stationary flow regimes with Rayleigh numbers 10⁴ ≤ Ra ≤ 10⁶. The finite volume approach solves the governing equations numerically. The findings show that the number of square obstacles within the square-shaped cavity significantly impacts heat exchange and hybrid nanofluid flow. The second example, with two square obstacles, improves heat exchange more than other cases with one to four barriers. In the second example, the obstacle location at the plane Y = 0.25H is suitable and helps boost heat transmission of the hybrid nanofluid. The ideal obstacle position in the fourth scenario, which has four square barriers, is at the plane Y = 0.75H.