Flow and Heat Transfer Characteristics of in-Line Impinging Jets With Cross-Flow At Short Jet-To-Plate Distance

The aim of this research is to numerically and experimentally study the flow and heat transfer characteristics of in-line impinging jets in cross-flow. The jets from a row of round orifices are perpendicularly impinged on the inner surface of a rectangular wind tunnel at a short distance between the orifice plate and impinged surface (H) of 2D, where D is a diameter of the orifice. The jet velocity was fixed corresponding to Re = 13,400 for all experiments, and the cross-flow velocity was varied at three different velocity ratios (velocity ratio, jet velocity/cross-flow velocity) of 3, 5, and 7. The heat transfer characteristic was visualized using a thermochromic liquid crystal sheet, and the Nusselt number distribution was evaluated by an image processing technique. The flow pattern on the impinged surface was also visualized by an oil film technique. The numerical simulation was used to explore a flow interaction between the impinging jets and cross-flow. The results indicated that Nusselt number peak increased by the increasing cross-flow velocity for short jet-to-plate distance. For the range determined, the maximum local Nusselt number peak was obtained at VR = 3 as the consequence of high velocity and high turbulence kinetic energy of jet impingement.     

Influence of Streamwise Pitch on Local Heat Transfer Distribution for In-Line Arrays of Circular Jets with Spent Air Flow in Two Opposite Directions

Abstract

The effect of streamwise jet-to-jet spacing on local heat transfer distribution due to an in-line rectangular array of confined multiple circular air jets impinging on a surface parallel to the jet plate are experimentally studied. The length-to-diameter ratio of nozzles of the jet plate is 1.0. The flow, after impingement, is constrained to exit in two opposite directions from the confined passage formed between the jet plate and target plate. Mean jet Reynolds numbers based on the nozzle exit diameter (d) covered are 3,000, 5,000, 7,500, and 10,000; jet-to-plate distances studied are d, 2d, and 3d. Streamwise jet-to-jet distances of 3d, 4d, and 5d, and a constant spanwise pitch of 4d, are considered. The jet plates have ten spanwise rows in the streamwise direction and six jets in each spanwise row. The flat heat transfer surface is made of thin stainless-steel metal foil. Local temperature distribution on a target plate is measured using a thermal infrared camera. Wall static pressures in the streamwise direction are measured midway between the spanwise jets to estimate cross-flow velocities and individual jet velocities. The streamwise distribution of the jet flow and the cross flow is found to be least influenced by the streamwise pitch variation for the range of parameters considered during the present study. Heat transfer characteristics are explained partially on the basis of flow distribution. The cooling performance, based on the strip-averaged Nusselt number per unit mass flow rate of coolant per unit area of cooled surface, indicates deterioration for lower streamwise pitch and higher jet-to-plate distance.     

Experimental Investigation of Heat Transfer for a Jet Impinging Obliquely on a Flat Surface

An experimental study investigated the effects of the inclination of an impinging circular jet on heat transfer from a horizontal flat plate. Local Nusselt number distribution was determine depending on inclination angle, jet-to-plate distance, and Reynolds number. The results show that the maximum heat transfer point moves toward the uphill side of the plate and that the maximum heat transfer decreases as the inclination angle decreases. The correlations were conducted to predict maximum and local Nusselt number as a function of Re, θ, L/D, and x/D for three specific regions.     

Influence of the Shape of the Orifice on the Local Heat Transfer Distribution of a Flat Plate Impinged by Under-Expanded Sonic Jets

Experiments are carried out for the circular, square, and equilateral triangular orifices of the same contraction ratio in order to explore the heat transfer characteristics for nozzle pressure ratios of 2.36, 3.04, 3.72, 4.4, and 5.08 at z/d = 2, 4, 6, and 8. The presence of vena contracta and shock cells and its locations are estimated from the impact pressure distribution along the axial direction. The pressure drop during the initial expansion is minimal for the equilateral triangular orifice. However, this pressure drop is comparable for the square and circular orifices. The heat transfer characteristics of the circular orifice are nearly axisymmetric, but that for non-circular jets are asymmetric, and the three dimensional effects are clearly observed. This is inferred through the spatial gradients of the Nusselt number and adiabatic wall temperature distributions. The linear correlations for the average Nusselt number are proposed for the circular, square, and equilateral triangular orifices.     

Heat Transfer Characteristics in a Double-Pipe Heat Exchanger Equipped with Coiled Circular Wires

An experimental investigation has been carried out to study the enhancement in heat transfer coefficient by inserting coiled wire around the outer surface of the inner tube of the double-pipe heat exchanger. Insulated wires, with a circular cross-section of 2 mm diameter, forming a coil of different pitches (p = 6, 12, and 20 mm), were used as turbulators. The investigation is performed for turbulent water flow in a double-pipe heat exchanger with cold water in the annulus space for both parallel and counter flows. The experiments were performed for Reynolds numbers ranging from 4,000 to 14,000. The experimental results reveal that the use of coiled circular wires leads to a considerable increase in heat transfer coefficients compared with a smooth wall tube for both parallel and counter water flows. The mean Nusselt number increases with Reynolds number and pitch. The convective heat transfer coefficient for a turbulent water flow increases for all coiled wire pitches, with the highest enhancement of about 450% for counter flow and 400% for the parallel flow. New correlations for mean relative Nusselt numbers at different coiled wire pitches are provided.     

Experimental Investigation of the Effect of Multiple Synthetic Jets on Heat Transfer and Pressure Loss in Minichannels

Synthetic jets can enhance thermal performance by creating wall impingement, disrupting boundary layers, and enhancing mixing within a minichannel. This study investigates using multiple synthetic jets along the length of minichannels with an aqueous–glycol mixture. Using synthetic jets, an average heat transfer coefficient increase up to 81% was demonstrated. It was also shown that pressure loss can be reduced by 78% while maintaining equivalent heat transfer performance. The effects of momentum ratio, frequency, and bulk Graetz number on heat transfer and pressure loss is presented along with an empirical correlation that predicts the heat transfer enhancement.     

水蒸气含量对高温平板气膜冷却传热影响研究

IGCC系统中新一代先进燃气轮机透平通流部分处于高温高湿的工作环境,因此其内部流动存在固有的复杂的流热耦合问题。为指导燃气轮机透平叶片冷却系统设计必须开展对流热耦合问题的试验与数值研究。本文在一平板气膜冷却流热耦合实验台上开展了模拟新一代燃气轮机透平通流部分高温高湿工作条件的流热耦合机理研究,并进行了数值模拟。研究工作...     

Flat Surface Heat Transfer Enhancement By An Impinging Circular Free Water Jet

Impinging a free water jet onto a heated flat plate was investigated with and without mesh screens. Increasing the jet velocity increased the surface cooling rate via turbulence increase. Lower temperature gradients existed due to the faster wetting front propagation for the water film to advance in shorter times. Decreasing the nozzle-to-block spacing slightly increased the cooling rates by counteracting effects of gravity acceleration and jet momentum dispersion. The turbulence role in the heat transfer enhancement was optimized via opposite changes in the strain rate and largest turbulence scale. Displacing the screens from the impingement zone increased the heat flux.     

Heat Transfer Enhancement from A Rectangular Flat Plate With Constant Heat Flux in Pulsating Flows

Experiments have been carried out to study heat transfer enhancement from a heated rectangular flat plate in pulsating flows. A heat transfer empirical formula of the heated rectangular flat plate in pulsating flows was developed that correlates the heat transfer enhancement factor to the Womersley number (α = 3.3–23.8), the Reynolds number (Re = 527–4,217), and the pressure coefficient (C _p  = 41.3–31,644.6). The results demonstrate that heat transfer from the rectangular flat plate was enhanced significantly under proper conditions. In addition, the influence of the Reynolds number on the heat transfer enhancement factor increases as the pressure amplitude increases.     

Influence of Burner Geometry on Heat Transfer Characteristics of Methane/Air Flame Impinging on Flat Surface

An experimental study has been conducted to find the heat transfer characteristics of methane/air flames impinging normally to a flat surface using different burner geometries. The burners used were of nozzle, tube, and orifice type each with a diameter of 10 mm. Due to different exit velocity profiles, the flame structures were different in each case. Because of nearly flat velocity profile, the flame spread was more in case of orifice and nozzle burners as compared to tube burner. Effects of varying the value of Reynolds number (600–2500), equivalence ratio (0.8–1.5) and dimensionless separation distance (0.7–8) on heat transfer characteristics on the flat plate have been investigated for the tube burner. Different flame shapes were observed for different impingement conditions. It has been observed that the heat transfer characteristics were intimately related to flame shapes. Heat transfer characteristics were discussed for the cases when the flame inner reaction cone was far away, just touched, and was intercepted by the plate. Negative heat fluxes at the stagnation point were observed when the inner reaction cone was intercepted by the plate due to impingement of cool un-burnt mixture directly on the surface. Different heat transfer characteristics were observed for different burner geometries with similar operating conditions. In case of tube burner, the maximum heat flux is around the stagnation point and decay is faster in the radial direction. In case of nozzle and orifice burner, the heat transfer distribution is more uniform over the surface.     

Heat Transfer in Oscillating Circular Pipes

An experimental investigation was made to study heat transfer in a pipe which is oscillated about an axis that is parallel to, but offset from, the pipe axis. Air was used as working fluid. The experimental setup was designed so as to provide oscillating motion of a test pipe. The measurement systems were installed on the oscillating section. For both steady and oscillating flows, the bulk air temperature and wall temperature, pressure drop, and frequency were measured. The parameters for this study were chosen as Reynolds number from 5,000 to 20,000 and oscillating frequencies from 10 to 20 Hz. The variations of Nusselt number versus these parameters were determined and presented graphically. Heat transfer enhancement of 42% was achieved at constant pumping power for oscillatory flow.     

撞击流中颗粒碰撞传热理论模型研究

建立了考虑颗粒接触面导热的颗粒碰撞传热模型,相应完善了气固两相撞击流理论模型,并对不同条件的撞击流干燥过程进行了模拟。计算结果与实验结果非常吻合,从而验证了所建模型的正确性。本文还分析了颗粒碰撞传热、加料方式以及喷嘴间距对撞击流干燥性能的影响规律。结果表明:在本文所涉及的撞击流干燥过程中,颗粒碰撞传热对干燥性能的影响并不明显;单边加料时颗粒物料的降水幅度及撞击流装置的干燥强度都明显大于双边加料的工况;喷嘴间距较大时,物料的降水幅度及装置的干燥强度均较大。     

Heat Transfer Performance through a Flat Tube Using Rotary Helical Screw-tape Inserts

Heat transfer characteristics and friction factor in the horizontal double pipes of a flat tube with full and varied spacer length helical screws at various rotational speeds are investigated for Reynolds numbers ranging from 580 to 1,582. The heat transfer and friction factor of the inserted tube are significantly increased compared to those of the plain tube. The Nusselt number, friction factors, and thermal enhancement efficiency were increased with decreasing spacer length at increasing rotational speed under the same operating conditions.     

Heat Transfer and Pressure Drop Characteristics in Turbulent Flow Through a Tube

An experimental investigation has been carried out for turbulent flow through a tube with perforated strip inserts. Strips were of mild steels with circular holes of different diameters. Flow varies, with ranging Reynolds numbers from 15,000 to 47,000. Air velocity, tube wall temperatures, and pressure drops were measured for a plain and strip-inserted tube. The heat transfer coefficient and friction factor were found to be 2.80 times and 1.8 times, respectively, that of the plain tube. The heat transfer performance was evaluated and found to be 2.3 times that of the plain tube based on constant blower power.     

裂解吸热反应对乙烷超临界传热的影响

本文对圆管内超临界状态下乙烷的湍流传热过程进行了数值模拟,分析了乙烷高温裂解对壁面热流密度、平均温度以及对流换热Nusselt数的影响。计算结果表明:考虑乙烷高温裂解吸热反应,管内平均温度会降低,出口处温度降低可达135 K;壁面热流密度则会显著增加,热出口处可增加近2倍;热裂解反应改善了超临界对流换热效果,Nusselt数可提高约20%。在本文计算条件下,经典的对流换热关系式可准确适用于不考虑热裂解反应的情况,而对于考虑裂解吸热反应的超临界乙烷传热情况则误差较大。     

Heat Transfer Study in a Discoidal System: The Influence of an Impinging Jet and Rotation

Abstract

This article presents an experimental study of the local heat transfer on the rotor surface in a discoidal rotor-stator system air-gap in which an air jet comes through the stator and impinges the rotor. To determine the surface temperatures, measurements were taken on the rotor, using an experimental technique based on infrared thermography. A thermal balance was used to identify the local convective heat transfer coefficient. The influence of the axial Reynolds number Re _j and the rotational Reynolds number Re was measured and compared with the data available in the literature. Local convective heat transfer coefficients were obtained for a dimensionless space between the two disks G = 0.01, for Re _j between 0 and 41,666, and for Re between 20,000 and 516,000. The flow data found in the literature can be used to explain the heat transfers in this small space configuration. In fact, the rotating disk can be divided into two influence zones: one dominated by the air jet near the center of the rotor and one affected by both the air jet and rotation. Heat transfers with non zero impinging jets appear to be continuously improved compared to those with no jets, even if the two influence zones mentioned previously are situated differently.     

Heat Transfer and Emission Characteristics of Impinging Rich Methane and Ethylene Jet Flames

Flame impingement heat transfer has widespread industrial and domestic applications and attaining high heat flux as well as low emission of pollutants is the important prerequisite for all such applications. In this article, the heat transfer and emission characteristics of a laminar flame jet impinging on a flat target plate have been investigated experimentally. The effect of reactant jet Reynolds number, equivalence ratio and burner to plate separation distance on the average heat flux, and emissions of CO and NO_x are studied using methane and ethylene fuels. Results indicate that the heat flux is maximized under certain operating conditions of jet Re, equivalence ratio, and separation distance between the burner and the target. Fuel type is found to have an effect on the heat transfer rate because of the varying luminosity of the flame with different fuels. Operating regimes that produce lower emission of pollutants are also identified. Findings of this article have direct industrial relevance to flame impingement heat transfer applications that have small target plate-to-burner port diameter ratios.     

Heat Transfer Analysis between R744 and HFOs inside Plate Heat Exchangers

Plate heat exchangers (PHE) are used for a wide range of applications, thus utilizing new and unique heat sources is of crucial importance. R744 has a low critical temperature, which makes its thermophysical properties variation smoother than other supercritical fluids. As a result, it can be used as a reliable hot stream for PHE, particularly at high temperatures. The local design approach was constructed via MATLAB integrated with the NIST database for real gases. Recently produced HFOs (R1234yf, R1234ze(E), R1234ze(Z), and R1233zd(E)) were utilized as cold fluids flowing through three phases: Liquid-phase, two-phase, and gas-phase. A two-step study was performed to examine the following parameters: Heat transfer coefficients, pressure drop, and effectiveness. In the first step, these parameters were analyzed with a variable number of plates to determine a suitable number for the next step. Then, the effects of hot stream pressure and cold stream superheating difference were investigated with variable cold channel mass fluxes. For the first step, the results showed insignificant differences in the investigated parameters for the number of plates higher than 40. Meanwhile, the second step showed that increasing the hot stream pressure from 10 to 12 MPa enhanced the two-phase convection coefficients by 17%, 23%, 75%, and 50% for R1234yf, R1234ze(E), R1234ze(Z), and R1233zd(E), respectively. In contrast, increasing the cold stream superheating temperature difference from 5 K to 20 reduced the two-phase convection coefficients by 14%, 16%, 53%, and 26% for R1234yf, R1234ze(E), R1234ze(Z), and R1233zd(E), respectively. Therefore, the R744 is suitable for PHE as a driving heat source, particularly at higher R744 inlet pressure and low cold stream superheating difference.     

含不同形状内热源的圆管内纳米流体自然对流及换热数值研究

采用格子玻尔兹曼方法对有三种恒温热源(圆形、三角形、方形)参与的圆管内纳米流体(铜-水)自然对流进行数值研究.主要研究瑞利(Ra)数,纳米颗粒体积分数以及热源几何形状等控制参数对纳米流体的流动与传热的影响.结果发现纳米颗粒体积分数的增加有利于强化传热,且在Ra数较小时,平均努塞尔(Nu)数增加的幅度要优于Ra数较大的情...     

Transient Heat Transfer for Forced Convection Flow of Helium Gas Over a Horizontal Plate

Transient heat transfer coefficients for helium gas flowing over a horizontal plate (ribbon) were measured under wide experimental conditions. The platinum plate with a thickness of 0.1 mm was used as the test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q₀ exp(t/τ). The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 290 to 353 K, and the periods of heat generation rate, τ, ranged from 50 ms to 17 s. The surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period τ longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlation for transient heat transfer was also obtained based on the experimental data.