Heat Transfer Characteristics Of Jet Research Articles

Area average heat transfer from a vertical flat plate impinged by circular inclined jet

ABSTRACT The inclined impinging jets owing to higher efficiency are widely used to disperse concentrated and transient heat loads. The temperature and heat transfer characteristics of jet cooling vertical hot plate at different inclinations are experimentally investigated. Experiments are performed to investigate the area average heat transfer over a flat surface of 600 × 300 mm2 placed at different location (z) of 3−20 d from the jet. The investigation is employed for Reynolds number based on hydraulic diameter of 11.5 mm ranging from 10,000 to 75,000. The orientation of the jet varies from 15° to 75°. The study aims to investigate the influence of jet orientation at different area locations on the surface from the point of impingement. The effect in stagnation region with different jet tilt is investigated. The area average temperature variation and characteristic temperature contours with increasing surface area and inclination is delineated. The characteristics of heat transfer on the surface’s periphery are studied. Heat transfer increases by 40% at lower tilt angle as the jet is displaced from 20 to 6 d, whereas the maximum average Nu occurs at higher inclination. The available Nusselt numbers in inclined jet are compared with present experimental results. A new empirical correlation is developed for a complete range of z/d, Re and large radial span.

Boiling heat transfer characteristics of pin-finned surface in distributed jet array impingement

In order to explore the effects of different jet positions in distributed jet array impingement boiling, a visualization study on pin-fin surfaces is conducted with HFE-7100 as the liquid coolant. The effects of three jet positions (Position I, II and III are the relative position for jet orifices locating in the middle of two pin-fins, on the top surface of pin-fins and in the middle of four pin-fins, respectively.) with different subcooling degrees, flow rates and jet distances on heat transfer characteristics of jet boiling are studied. Meanwhile, the effects of jet positions on bubble dynamics and pressure drop of jet impingement boiling are investigated with emphasis. For each jet position, increasing the subcooling all decreases the onset of nucleate boiling (ONB). Moreover, because of the dominant role of nucleate boiling, the flow rate has little effect on heat transfer coefficient in fully-developed nucleate boiling regimes. The effects of jet distances on heat transfer are different for three jet positions, but there should be an optimum jet distance to achieve the best heat transfer performance. It is found that there are basically no differences in the cooling performance of the three jet positions in single-phase heat transfer regime, while different jet positions principally affect the boiling heat transfer. Meanwhile, different jet positions have similar heat transfer behaviors in the fully boiling regime. Different heat transfer characteristics of jet boiling for three jet positions are induced by different velocity distribution around pin fins and bubbles exhausted effectively or not.

Investigation of the transient heat transfer to a supersonic air jet impinging on a high-temperature plate based on a discrimination-experiment method.

A discrimination-experiment method is developed to investigate the transient heat transfer of air jet impingement by discretizing the solid domain into mutually adiabatic test cylinders. This method can not only reduce the influence of the transverse heat transfer of a solid domain on the heat transfer characteristics of the jet but can also simplify the two-dimensional or three-dimensional heat conduction problem into a one-dimensional problem. Moreover, the discrimination-experiment method eliminates the embedment of thermocouples into the solid domains, further improving the accuracy and reliability of the proposed method. The transient heat transfer characteristics of a supersonic air jet impinging on a high-temperature target (860°C) and the effects of thermo physical parameters, such as the density, specific heat capacity, thermal conductivity and nozzle-to-target distance (H/D = 3, 4, and 5) are analyzed in detail using the discrimination-experiment method. The results provide important guidance for the thermal design of supersonic air jet impingement.

On heat transfer and flow characteristics of jets impinging onto concave surface with varying bleeding arrangements

Purpose The purpose of this study is to investigate the effects of film holes’ arrangements and jet Reynolds number on flow structure and heat transfer characteristics of jet impingement conjugated with film cooling in a semicylinder double wall channel. Design/methodology/approach Numerical simulations are used in this research. Streamlines on different sections, skin-friction lines, velocity, wall shear stress and turbulent kinetic energy contours near the concave target wall and vortices in the double channel are presented. Local Nusselt number contours and surface averaged Nusselt numbers are also obtained. Topology analysis is applied to further understand the fluid flow and is used in analyzing the heat transfer characteristics. Findings It is found that the arrangement of side films positioned far from the center jets helps to enhance the flow disturbance and heat transfer behind the film holes. The heat transfer uniformity for the case of 55° films arrangement angle is most improved and the thermal performance is the highest in this study. Originality/value The film holes’ arrangements effects on fluid flow and heat transfer in an impingement cooled concave channel are conducted. The flow structures in the channel and flow characteristics near target by topology pictures are first obtained for the confined film cooled impingement cases. The heat transfer distributions are analyzed with the flow characteristics. The highest heat transfer uniformity and thermal performance situation is obtained in present work.

Jet A-1 연료의 열전달 특성에 관한 실험적 연구

Jet A-1 연료의 열전달 특성에 관한 실험적 연구

On heat transfer and flow characteristics of jets impinging onto a concave surface with varying jet arrangements

The increase in hot gas temperature is helpful for the turbine efficiency improvement and energy-saving. The significantly curved leading edge suffers the highest thermal load in a turbine blade. Jet impingement is one of the popular heat transfer enhancement methods, which has been widely used in blade leading edge. In this study, the flow structure and heat transfer characteristics of jets impinging onto a curved surface with varying jet arrangements and Reynolds number (10,000–40,000) are numerically investigated. The relative jet-to-target spacing equals 1, and relative surface curvature equals 10. An array jets arrangement is provided as baseline. Concerning three array cases, jet holes are positioned in inline and staggered patterns with changing jet-to-jet spacing. In this work, streamlines of different sections, limiting streamlines near target wall and vortex, are obtained. Local Nusselt number contour, local Nusselt number curves and surface-averaged Nusselt number are also presented. Local heat transfer characteristics are analyzed with fluid flow. It is also shown that the heat transfer uniformity of both inline and staggered cases is significantly enhanced by comparing with an array jets case. The whole curved surface-averaged Nusselt number increases with increasing jet-to-jet streamwise spacing at inline arrangement.

Irradiation Target Cooling Using Circular/Slot Air Jet

ABSTRACTTo study the effect of irradiation on materials, sample coupons are irradiated in cyclotron facilities. During the irradiation process, these samples produce significant heat. This heat needs to be continuously removed from the samples in order to avoid melting of the samples as well as to keep the samples at a particular temperature during irradiation. The area available for heat transfer is limited due to small size of the samples. To increase the heat transfer rate, jet cooling is used as it provides large heat transfer co-efficient. To understand the heat transfer characteristics of jet cooling under these conditions, experiments have been carried out. Two inclined jets hitting on both sides of the target plate give maximum cooling and uniform temperature distribution. This paper gives the details of the numerical and experimental studies carried out and the discussions about the results obtained.

Effect of jet-mainstream velocity ratio on flow characteristics and heat transfer enhancement of jet on flat plate flow

The aim of this research was to study the effect of jet-mainstream velocity ratio on flow and heat transfer characteristics of jet on flat plate flow. The jet from pipe nozzle with inner diameter of D=14 mm was injected perpendicularly to mainstream on flat plate. The flat plate was blown by mainstream with uniform velocity profile at 10 m/s. The velocity ratio (jet to mainstream velociy) was varied at VR=0.25 and 3.5 by adjusting velocity of jet flow. For heat transfer measurement, a thin foil technique was used to evaluate the heat transfer coefficient by measuring temperature distributions on heat transfer surface with constant heat flux by using infrared camera. Flow characteristics were simulated by using a computational fluid dynamics (CFD) with commercial software ANSYS Fluent (Ver.15.0). The results showed that the enhancement of heat transfer along downstream direction for the case of VR=0.25 was from the effect of jet stream whereas for the case of VR=3.5 was from the effect of mainstream.

Cooling of small size irradiation specimens using impinging jets

To study the effect of irradiation on materials, sample coupons are irradiated in cyclotron facilities. During the irradiation process, these samples produce significant amount of heat. This heat needs to be continuously removed from the samples in order to avoid melting of the samples as well as to keep the samples at a particular temperature during the irradiation process. The area available for heat removal is limited due to the small size of the samples. To increase the heat removal rate, jet cooling is employed as it provides large heat transfer co-efficient. To understand the heat transfer characteristics of jet cooling under these conditions, experiments have been carried out. Electric Joule heating is adopted to simulate irradiation heat in stainless steel samples. Circular / slot nozzles are used to create air jet. From the study the values of the parameters correspond to the maximum heat removal rate are found out. The results are also compared with empirical correlations from the literature.

Micro device design and fabrication for the experimental investigation of jet impingement on an array of micro pin fins

This work presents the design, fabrication, and demonstration of micro devices for the experimental investigation of heat transfer characteristics of jets impinging on an array of micro pin fins for high heat flux microelectronics and photonics cooling applications. The micro devices, fabricated using MEMS fabrication techniques, consist of an array of radially spaced micro pin fins with diameters ranging from 50 µm–125 µm, height of 230 µm, pitch of 250 µm, and a base area of 2 × 2 mm2. The micro pin fins are of different geometries, namely circular, elliptical, hydrofoil, square, and rectangular. Single-phase and boiling heat transfer experiments were carried out using refrigerant R134a. Enhancement of both the single-phase and the two-phase heat transfer coefficients were observed. Due to the presence of the micro pin fins, the boiling heat transfer was characterized by the suppression of boiling hysteresis. With a better heater design, the proposed micro device is an excellent platform for studying jet impingement heat transfer on micro pin fins.

LBE simulation of coherent vortex motion and heat transfer in jets of cross flow

Lattice Boltzmann equation method is used to simulate the coherent vortex motions and interactions and the heat transfer characteristics of jets in cross flow (JICFs) via TD2G9 model. After validation, the characteristics of cross flow under different Reynolds numbers are illustrated, including the mean profiles, the Reynolds stress tensor, the vortex and temperature fields, the temperature gradients near the walls, and the coherent correlation of vortex motions. The results show that the velocity profiles in JICF can be characterized by three basic regions, which are mainly caused by the mergence of JICF with the main flow. The temperature gradient near the walls can also be categorized by four basic regions, which are caused mainly by the impulse of JICFs too. Coherent vortex motions are found in JICF for Re = 3000, which are proved by strong periodic correlation of flow variables over a fixed area.

Recent Trends in Computation of Turbulent Jet Impingement Heat Transfer

A review of the current status of computation of turbulent impinging jet heat transfer is presented. It starts with a brief introduction to flow and heat transfer characteristics of jet impinging flows considering the simplest jet impinging geometry: normal impingement of a single jet into a flat surface. Subsequently, a review of recent computational studies related to the same geometry is presented. The effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail. A review of direct numerical simulation of the same geometry is also presented. Further, some recent attempts in Reynolds-averaged Navier–Stokes modeling of impinging flows are also reviewed. A review of computation of other complex impinging flows is also presented. The review concludes with a listing of some important findings and future directions in the computation of impinging flows.