Heat Transfer Characteristics Of Impingement Research Articles

Heat Transfer Characteristics of Cooling High Temperature Steel Plate by Single Round Jet Impingement

The heat transfer characteristics of a single round air jet impingement on a high temperature steel plate were examined experimentally using a single‐point temperature measurement method, incorporated with solving the inverse heat conduction problem. During the experiments, the temperature of the steel plate varied from 1073 K to 373 K, the Reynolds number was set to 27,000, the nozzle to plate spacing was set to 4. The results indicated that the radial distribution of the local Nusselt number is bell‐shaped at the initial stage of the transient cooling process. As the cooling process continues, the local Nusselt numbers decrease and a second peak occurs at r/D = 2. The area averaged Nusselt number are in accordance with the correlation proposed by Hofmann and Martin at first and then decrease significantly, but this trend is not obvious at r/D > 10.

Correlations for boiling heat transfer characteristics of high-velocity circular jet impingement on the nano-characteristic stagnation zone

The jet impingement boiling is the most effective heat transfer method among all the steady-state heat transfer forms. However, various researches on the jet impingement boiling still lack experimental data for heat transfer characteristics under high impact velocity and special heated surface topography. Due to this reason, the present experimental study investigates the heat transfer characteristics of high-velocity circular jet impingement boiling on the nano-characteristic surface of the stagnation zone with different surface wettability. The wettability of the copper surface is varied to hydrophilic or hydrophobic nano-characteristic surface by modifying surface topography and chemistry. The effects of impact velocity, subcooling and solid–liquid contact angle (CA) on the onset of nucleate boiling (ONB), nucleate boiling heat transfer coefficient (HTC) and critical heat flux (CHF) are studied experimentally. It is found that the effects of impact velocity, subcooling and CA on the ONB are respectively independent. Comprehensive effects of impact velocity and CA on the nucleate boiling HTC are also obtained. The proportion of nucleate boiling HTC after the ONB point gets more significant with increasing wall superheat. The HTC will increase with increasing impact velocity and CA. Some empirical correlations are proposed for predicting the effects of impact velocity, subcooling and CA on the jet boiling heat transfer characteristics. What’s more, the method for predicting the wall superheat corresponding to the CHF is discussed. The comparison results indicate the predicted value agrees well with the experimental data.

Effects of pin fin shape and configuration on the single-phase heat transfer characteristics of jet impingement on micro pin fins

Experiments to investigate the effects of cross flow area and pin fin shapes on the single-phase impingement point heat transfer coefficients of jet impingement on micro pin fins have been performed. The micro pin fins were circular, hydrofoil, square, and elliptical pin fins with characteristic lengths ranging from 50μm to 125μm. Jet parameters were fixed with an orifice diameter of 2.0mm and a stand-off ratio of 0.865. Using R134a as the working fluid, Reynolds numbers based on jet diameter were varied from 8000 to 80,000. Cross flow effects were found to have little to no significance on the heat transfer coefficients while area enhancement played a major role on the overall heat transfer enhancement. When comparing the various micro pin fins, the circular pin fins along with the square pin fins displayed the highest heat transfer coefficients for a given jet velocity.

Numerical simulation and optimization of impingement cooling for rotating and stationary pin–fin heat sinks

The turbulent fluid flow and heat transfer characteristics of air jet impingement onto the rotating and stationary heat sink have been investigated numerically and optimized using genetic algorithms (GAs). The squared heat sinks with uniform 5×5 pin–fins are employed. The turbulent governing equations are solved by using the finite volume method combined with the approaches of four different turbulent models based on the Reynolds-averaged Navier–Stokes (RANS). The relative performance of four turbulent models for predicting this type of flow and heat transfer is investigated by comparing the numerical results with available experimental data. It is found that the standard k–ɛ model can give predictions for better performance of fluid flow and heat transfer.Studied parameters are included the distance of nozzle to fin tip (0⩽C/d⩽11), Reynolds number (5019⩽Re⩽25,096) and rotational Reynolds number (0⩽Rer⩽8114). It is found that Nu0‾ increases with Re for a stationary heat sink. The effects of Rer on the average Nusselt number (NuΩ‾) of a rotating heat sink with jet impingement demonstrate that heat transfer enhancement (NuΩ‾/Nu0‾) is obvious in the case of smaller Re (Re=5019), but NuΩ‾/Nu0‾ decreased with increasing Re. In addition, the optimization of this problem is also presented by using response surface methodology (RSM) and genetic algorithm (GA) method. Three design variables including the distance of nozzle to fin tip, fin height and fin width are selected for optimization. Based on the results, the optimum condition is C/d=0, w/d=0.77 and Hf/d=3.08 for a stationary and a rotating pin–fins heat sink.

Nanofluid jet impingement heat transfer characteristics in the rectangular mini-fin heat sink

The nanofluid jet impingement heat transfer characteristics in a rectangular mini-fin heat sink are studied. The heat sink is fabricated from aluminum by a wire electrical discharge machine. The nanofluid is a mixture of deionized water and nanoscale TiO2 particles with a volume nanoparticle concentration of 0.2%. The results obtained for nanofluid jet impingement cooling in the rectangular mini-fin heat sink are compared with those found in the water jet impingement cooling. The effects of the inlet temperature of the nanofluid, its Reynolds number, and the heat flux on the heat transfer characteristics of the rectangular mini-fin heat sink are considered. It is found that the average heat transfer rates for the nanofluid as coolant are higher than those for deionized water.

Flow Visualization and Heat Transfer Characteristics of Liquid Jet Impingement

Equipment used to cool horticultural produce often involves three-phase porous media. The flow field and heat transfer processes that occur in such equipment are generally quantified by means of empirical relationships amongst dimensionless groups. This work represents a first step towards the goal of harnessing the power of computational fluid dynamics (CFD) to better understand the heat transfer processes that occur in beds of irrigated horticultural produce. The primary objective of the present study is to use numerical predictions towards reducing the energy and cooling water requirement in cooling horticultural produce. In this paper, flow and heat transfer predictions are presented of a single slot liquid jet impinging on flat and curved surfaces using a CFD code (FLUENT) for 2D configurations. The effects of Reynolds number, nozzle to plate spacing, nozzle width, and target surface configuration have been studied. Reynolds numbers of 250, 375, 500, 700, 1000, 1500, 1800, and 1900 are studied where the liquid medium is water. Here, the Reynolds number is defined in terms of the hydraulic nozzle diameter, inlet jet velocity, and fluid kinematic viscosity. The results show that Reynolds numbers, nozzle to plate spacing, and nozzle width have a significant effect on the flow field and heat transfer characteristics, whereas the target surface configuration at the stagnation area has no substantial impact. The use of a numerical tool has resulted in a detailed investigation of these characteristics, which has not been available in the literature previously.

Numerical simulation of flow and heat transfer from slot jets impinging on a cylindrical convex surface

Flow and heat transfer characteristics of slot jets impingement to a cylindrical convex surface are numerically investigated. Suitable turbulence models have been determined through comparison with the experimental data. Flow structures are described and impingement heat transfer characteristics are discussed. The effects of Re, H/B and D/B on single-slot jets impingement heat transfer are analyzed and heat transfer characteristics of multiple-slot jets are investigated. The results show that: Gas flows along the convex surface and boundary layer separation occurs in both single and multiple-slot jets impingement. A maximum stagnation Nu appears at H/B=8 and the local Nu decreases with increasing H/B in the region far away from the stagnation. The Nu in the stagnation region decreases with increasing D/B but the Nu is nearly the same in the region far away from the stagnation. Pressure gradient is an important factor on heat transfer enhancement. Correlations of the Num for single-slot, double-slot and quadric-slot jets impinging on a convex surface are obtained. It indicates the effects of Re and D/B on Num could become more important in less slot jets impingement.

Effects of outlet port positions on the jet impingement heat transfer characteristics in the mini-fin heat sink

Effects of outlet port positions on the jet liquid impingement heat transfer characteristics in the mini-rectangular fin heat sink are numerically investigated. The three-dimensional governing equations for fluid flow and heat transfer characteristics are solved using finite volume scheme. The standard k-ε turbulent model is employed to solve the model for describing the heat transfer behaviors. The predicted results obtained from the model are verified by the measured data. The predicted results are reasonable agreement with the measured data. The outlet port positions have significant effect on the uniformities in velocity and temperature. Based on the results from this study, it is expected to lead to guidelines that will allow the design of the cooling system to ensure the electronic devices at the safe operating temperature.

Experimental Study of Jet Nanofluids Impingement System for Cooling Computer Processing Unit

An experimental investigation of the jet nanofluids impingement heat transfer characteristics of mini-channel heat sink for cooling computer processing unit of personal computer is performed. The experiments are tested under the real personal computer operating conditions: no load and full load conditions. The experiments are performed for the following ranges of the parameters: coolant flow rate varies from 0.008 to 0.020 kg/s, the nozzle diameter is set to 1.00, 1.40, 1.80 mm, the distance nozzle-to-fins tip is 2.00 mm, the channel width of the mini-channel heat sink is 1.00 mm. The nanofluids with suspending of TiO2 particles in base fluid are used as a working fluids. It was observed that the average CPU temperatures obtained from the jet nanofluids impingement cooling system are 3.0%, 6.25% lower than those from the jet liquid impingement and from the conventional liquid cooling systems, respectively. However, this cooling system requires higher energy consumption.

Investigation on the jet liquid impingement heat transfer for the central processing unit of personal computers

In this paper, the jet liquid impingement heat transfer characteristics in the mini-rectangular fin heat sink for the central processing unit of a personal computer are experimentally investigated. The experiments are tested with three different channel width heat sinks under real operating conditions: no load and full load conditions. The jet liquid impingement cooling with mini-rectangular fin heat sink system is introduced as the active and passive heat transfer enhancement techniques. Effects of relevant parameters on the central processing unit temperature are considered. It is found that the central processing unit temperatures obtained from the jet liquid impingement cooling system are lower than those from the conventional liquid cooling system; however, the energy consumption also increases. The results of this study are of technological importance for the efficient design of cooling systems of personal computers or electronic devices to enhance cooling performance.

Heat Transfer Analysis in a Rectangular Duct Without and With Cross-Flow and an Impinging Synthetic Jet

A synthetic jet is a zero-net-mass-flux device, which synthesizes stagnant air to form a jet, and is potentially useful for cooling. Due to the inherent suction and ejection processes in a synthetic jet, its utility in a confined enclosure is not obvious. The synthetic jet impingement heat transfer characteristics inside a rectangular duct are studied in this paper. In addition, the effect of cross-flow created using either fans or another synthetic jet on its heat dissipation capability is examined. Experiments are conducted for different jet Reynolds numbers (Re), in the range of 950-4000, at different offset positions of the synthetic jet with respect to a heated block flush mounted on one surface of the duct. The height of the duct is the same (25 mm) for all measurements while the width is varied between 110 mm and 330 mm in order to examine the effect of confinement on the heat transfer coefficient. The change in the width of the duct is found to have a negligible effect on heat transfer. The heat transfer coefficient is found to be more with synthetic jet direct impingement (150 W/m2 · K) than with combined flow (both impingement and cross-flow) (134 W/m2 · K) or with only cross-flow (45 W/m2 · K) in the duct. The offset of the synthetic jet from the center of the heated block is found to drastically reduce the heat transfer. These results are expected to be useful for designing synthetic jet-based cooling solutions.

Heat transfer characteristics of synthetic jet impingement cooling

Synthetic jet is a novel flow technique which synthesizes stagnant air to form a jet, and is potentially useful for cooling applications. The impingement heat transfer characteristics of a synthetic jet are studied in this work. Toward that end, the behavior of the average heat transfer coefficient of the impinged heated surface with variation in the axial distance between the jet and the heated surface is measured. In addition, radial distribution of mean and rms velocity and static pressure are also measured. The experiments are conducted for a wide range of input parameters: the Reynolds number (Re) is in the range of 1500–4200, the ratio of the axial distance between the heated surface and the jet to the jet orifice diameter is in the range of 0–25, and the length of the orifice plate to the orifice diameter varies between 8 and 22 in this study. The maximum heat transfer coefficient with the synthetic jet is found to be upto 11 times more than the heat transfer coefficient for natural convection. The behavior of average Nusselt number is found to be similar to that obtained for a continuous jet. The exponent of maximum Nusselt number with Re varies between 0.6 and 1.4 in the present experiments, depending on the size of the enclosure. A direct comparison with a continuous jet is also made and their performances are found to be comparable under similar set of conditions. Such detailed heat transfer results with a synthetic jet have not been reported earlier and are expected to be useful for cooling of electronics and other devices.

Experimental Study of Flow Structures of Circular Pulsating Air Jet

Problem Statement: Applications of impingement jets in industry for heating and cooling purposes requires a high convective heat transfer coefficient. Numerous studies have been conducted to improve the convective heat transfer coefficient for a steady impinging jet. A pulsating jet has a very high potential in replacing steady jet after it been found able to increase the heat transfer coefficients at certain pulsating frequencies. The objectives of this study were to; (i) determine the velocity profile of a circular pulsating air jet at different pulse frequencies and Reynolds Number using a rotating valve pulse jet system and (ii) to compare the normalized steady and pulsed jet velocity at highest Reynolds number of 32 000 and highest pulsating frequency of 80Hz. Approach: Pulsation of the air jet was produced by a rotating cylinder valve mechanism at frequencies between 10-80 Hz. Flow structures of the heated steady and pulse single circular axisymmetric air jet velocity were measured using a calibrated hot-wire anemometer and presented in non-dimensional form. The measurements were carried out at three different Reynolds numbers which was set at 16000, 23300 and 32000. The jet exit velocity profile for all the test frequency is determined by plotting the graph of radial distance against the non-dimensional jet exit velocity. Results: The corresponding Reynolds number in this test is based on time-averaged centerline velocity. The results of the velocity measurement were plotted side by side using non-dimensional parameters in order to make direct comparison of the velocity profile at different frequencies and Reynolds numbers. Stagnation point velocities are the same for steady and pulsating jet for all pulse frequencies. As the radial distance from the stagnation point increases, pulsating velocity increases between 20-30% from radial distance of 2-22 mm. Conclusion: Results of the flow structures plotted show a distinctive exit air jet profile which can affect the impingement heat transfer characteristics. This was the result of enhanced turbulence intensity due to pulsating jet produced by the rotating cylinder. From the jet exit velocity profile obtained, it is found that mass flow rate for different test frequencies are slightly different due to the difference in the local velocity measurement affected by the pulses. The jet exit velocity profile data will be used to form a correlation between the pulsating jet velocity and heat transfer data.

1807 ミニチャンネル管群による衝突噴流熱伝達の特徴(G06-2 伝熱(2),G06 熱工学)

1807 ミニチャンネル管群による衝突噴流熱伝達の特徴(G06-2 伝熱(2),G06 熱工学)

0410 閉鎖空間における単孔衝突噴流の伝熱特性に関する研究(S47 次世代ガスタービン,S47 次世代ガスタービン)

0410 閉鎖空間における単孔衝突噴流の伝熱特性に関する研究(S47 次世代ガスタービン,S47 次世代ガスタービン)

612 脈動空気噴流の衝突熱伝達特性 : 衝突板上流に設置した多孔板の効果(熱工学II)

612 脈動空気噴流の衝突熱伝達特性 : 衝突板上流に設置した多孔板の効果(熱工学II)

Velocity measurements and flow structure visualizations of a self-sustained oscillating jet

The purpose of this study is the experimental investigation on self-sustained oscillating jet characteristics. Main aim was to describe vertical structures of turbulent air jet issuing from the nozzle of special configuration modified by the controlled oscillations in free jet setup. In the present experiments was used so-called "whistler nozzle", a simple-structured device capable to induce self-sustained excitations with controllable frequencies depending on the nozzle geometrical configuration. The frequency of the excitation measured with a far-field condenser microphone probe was around 1-2 kHz. The jet Reynolds number was in the range 48,000-95,000 in all experimental conditions presented in this paper. Flow field velocity measurements were provided in the free jet setup, with and without self-sustained excitations. The images of both free and impinging jets were taken with a high-speed digital video camera. The flow field and structure of the jet were found to be extremely sensitive to the excitation and dependent on the excitation conditions. This fact can lead to the conclusion that the local heat transfer characteristics of jet impingement are also remarkably dependent on the jet excitation.

Critical Heat Flux of Steady Boiling for Water Jet Impingement in Flat Stagnation Zone on Superhydrophilic Surface

The nucleate boiling heat transfer characteristics of a round water jet impingement in a flat stagnation zone on the superhydrophilic surface were experimentally investigated. The superhydrophilic heat transfer surface was formed by a TiO2 coating process. The experimental results were compared with those on the common metal surface. In particular, the quantificational effects of the flow conditions, heating conditions, and the coating methods on the critical heat flux (CHF) were systemically investigated. The experimental data showed that the nucleate boiling heat transfer characteristics on the superhydrophilic surface are significantly different from those on the common metal surface. The CHF of boiling on the superhydrophilic surface is greatly increased by decreasing of the solid-liquid contact angle.

A223 干渉を伴う非円形衝突二噴流熱伝達に及ぼす音場の影響(OS-7&8 対流による熱伝達特性制御II)

The impinging jet has been used in many industrial fields, such as, the heating, cooling and drying of glass, paper, fiber, and steel. Recently, it is shown that tone-excitation gives a remarkable variation on thermal and pressure fields on the target plate, when its tone-excitation with a specific frequency for an oval jet is vertically added in the direction of the flow of the jet. This report examined the effect of tone-excitation on the interference of free jet, and the impingement heat transfer characteristics from dual jets with non-circular orifice.

614 脈動水噴流の流動と衝突熱伝達特性(熱工学III)

614 脈動水噴流の流動と衝突熱伝達特性(熱工学III)