Maximum heat flux propagation velocity during quenching by water jet impingement

Maximum heat flux propagation characteristics during quenching of hot cylindrical blocks with initial temperature 250–600 °C have been investigated experimentally using a subcooled water jet. When the wetted area starts moving towards the circumferential region, the heat flux reaches its maximum value and the position of maximum heat flux follows the visible leading edge of the wetting front. If wetting starts immediately after the jet strikes the surface, the velocity of this maximum heat flux point increases with the increase of jet velocity and subcooling and decreases with the increase of block initial temperature. These trends are opposite if there is a long delay before movement of the front.     

Maximum heat flux in relation to quenching of a high temperature surface with liquid jet impingement

Experimental investigation has been conducted for quenching of hot cylindrical blocks made of copper, brass and steel with initial block temperature 250–400 °C by a subcooled water jet of diameter of 2 mm. The subcooling was from 5 to 80 K and the jet velocity was from 3 to 15 m/s. After impingement, the jet stagnates for a certain period of time in a small region near the centre and then the wetting front starts moving outwards. During this movement, when the surface temperature at the wetting front drops to 120–200 °C, the surface heat flux reaches its maximum value due to forced convection nucleation boiling. The maximum heat flux is a strong function of the position on the hot surface, jet velocity, block material properties and jet subcooling. A new correlation for maximum heat flux is proposed.     

Delay of wetting propagation during jet impingement quenching for a high temperature surface

Transient heat transfer has been investigated experimentally with a subcooled water jet during quenching of hot cylindrical blocks made of copper, brass and steel for initial surface temperatures from 250 to 400 °C. The jet velocity was from 3 to 15 m/s and jet subcooling from 5 to 80 K with a jet diameter of 2 mm. When the jet strikes the hot surface, the wetting front becomes stagnant for a certain period of time in a small central region before wetting the entire surface. This wetting delay may be described as the resident time which is a strong function of block material and jet subcooling and also a function of initial block temperature and jet velocity. New correlations for the resident time and the surface temperature at the resident time at the wetting front have been proposed.     

Correlation of critical heat flux in hybrid jet impingement/micro-channel cooling scheme

Experiments were performed to investigate the two-phase cooling characteristics of a new hybrid cooling scheme combining the cooling attributes of slot jets and micro-channel flow. A test module was constructed in which dielectric PF-5052 liquid was introduced through five 0.48 mm wide and 12.7 mm long slot jets, each leading to a 1.59 mm wide and 1.02 mm deep channel. Increases in flow rate and subcooling yielded similar trends of delaying the inception of boiling and increasing critical heat flux (CHF). A previous channel flow correlation predicted CHF values far smaller than measured, while those for slot jets yielded closer predictions. This proves the cooling performance of the hybrid configuration is dominated more by jet impingement than by micro-channel flow. By dividing the test surface into a portion that is dominated by jet impingement and another by micro-channel flow, and applying the appropriate CHF correlation for each portion, the CHF data for this hybrid cooling configuration are predicted with a mean absolute error of 8.42%.     

Estimation of heat flux on the surface of an initially hot cylinder cooled by a laminar confined impinging jet

In this study, an inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied to estimate the unknown space-and time-dependent heat flux at the surface of an initially hot cylinder cooled by a laminar confined slot impinging jet from the knowledge of temperature measurements taken on the cylinder’s surface. It is assumed that no prior information is available on the functional form of the unknown heat flux; hence the procedure is classified as the function estimation in inverse calculation. The temperature data obtained from the direct problem are used to simulate the temperature measurements, and the effect of the errors in these measurements upon the precision of the estimated results is also considered. The results show that an excellent estimation on the space-and time-dependent heat flux can be obtained even the distributions of thermal properties inside the cylinder is unknown.     

Effect of CNT concentration and agitation on surface heat flux during quenching in CNT nanofluids

In this article, the effect of Carbon Nanotube (CNT) concentration and agitation on the heat transfer rate has been studied during immersion quenching in CNT nanofluids. For this purpose, CNT nanofluids were prepared by suspending chemically treated CNTs (TCNT) at four different concentrations in deionized (D.I) water without using any surfactant. Quench probes with a diameter of 20 mm and a length of 50 mm were machined from 304L stainless steel (SS) and quenched in water and CNT nanofluids with the CNT concentration ranging from 0.25 to 1.0 wt.%. The heat flux and temperature at the quenched surface were estimated based on the Inverse Heat Conduction (IHC) method using the temperature data recorded at 2 mm below the probe surface during quenching. The computation results showed that the peak heat flux increased with an increase in the CNT concentration up to 0.50 wt.% and started decreasing with further increase in the CNT concentration. The enhanced heat transfer performance of CNT nanofluids during quenching at lower concentration of CNTs is attributed to their higher effective thermal conductivity. The reduced heat transfer performance of CNT nanofluids having higher concentration of CNTs is due to the increased viscosity of CNT nanofluids. The effect of agitation on heat transfer rate during quenching has also been studied in this work by stirring the CNT nanofluid prepared with 0.50 wt.% of CNTs which recorded the maximum peak heat flux among the four concentrations. The effect of CNT nanofluid agitation was counter-intuitive and resulted in decreased heat transfer rate with the increase in agitation rate.     

Effect of crossflow regulation by varying jet diameters in streamwise direction on jet impingement heat transfer under maximum crossflow condition

Jet impingement heat transfer has been studied numerically for a maximum crossflow condition using a 3?×?9 array of jets. Five-hole configurations have been studied for jet average Reynolds numbers ranging from 10,000 to 20,000. Crossflow has been mitigated by varying the jet diameters in the streamwise direction to reduce the impact of crossflow on downstream jet impingement. The design criteria for all five configurations were to keep the average of the jet diameters equal to the constant jet diameter configuration (baseline). It has been found that the configuration with increasing and then decreasing jet diameters provided higher levels of heat transfer with more uniform cooling when compared to the traditional constant diameter configuration and other configurations.     

Mixed convection on jet impingement cooling of a constant heat flux horizontal porous layer

In the present study, numerical investigation of jet impingement cooling of a constant heat flux horizontal surface immersed in a confined porous channel is performed under mixed convection conditions, and the Darcian and non-Darcian effects are evaluated. The unsteady stream function-vorticity formulation is used to solve the governing equations. The results are presented in the mixed convection regime with wide ranges of the governing parameters: Reynolds number (1 ≤ Re ≤ 1000), modified Grashof number (10 ≤ Gr¹ ≤ 100), half jet width (0.1 ≤ D ≤ 1.0), Darcy number (1 × 10^?6 ≤ Da ≤ 1 × 10^?2), and the distance between the jet and the heated portion (0.1 ≤ H ≤ 1.0). It is found that the average Nusselt number (Nu_avg) increases with increase in either modified Grashof number or jet width for high values of Reynolds number. The average Nusselt number also increases with decrease in the distance between the jet and the heated portion. The average Nusselt number decreases with the increase in Da for the non-Darcy regime when Re is low whereas Nu_avg increases when Re is high. It is shown that mixed convection mode can cause minimum heat transfer unfavorably due to counteraction of jet flow against buoyancy driven flow. Minimum Nu_avg occurs more obviously at higher values of H. Hence the design of jet impingement cooling through porous medium should be carefully considered in the mixed convection regimes.     

On the impingement heat transfer of an oblique free surface plane jet

The objective of the present study is to understand the hydrodynamics and heat transfer of the impingement process, particularly the complexities attributable to the asymmetric geometry of an oblique liquid plane jet. The Navier-Stokes equations are solved using a finite-volume formulation with a two-step projection method on a fixed non-uniform rectangular grid. The free surface of the jet is tracked by the volume-of-fluid method with a second order accurate piecewise-linear scheme. The energy equation is modeled by using an enthalpy-based formulation. The method provides a state-of-the-art comprehensive model of the dynamic and thermal aspects of the impinging process. Nusselt number plots and pressure distributions on the substrate are obtained. The locations of the maximum Nusselt number as well as maximum pressure on the surface are identified and compared with the geometric jet impingement point. Results for normal impingement are also obtained and are used as reference. The effects of several parameters are examined. These include jet Reynolds number, jet impingement angle and jet inlet velocity profile. Experimental and analytical data from the literature are also included for comparison.     

Enhancement of an impingement heat transfer between turbulent mist jet and flat surface

The work presents the results of numerical investigation of the flow structure and heat transfer of impact mist jet with low concentration of droplets (M_L1 ? 1%). The downward gas-droplets jet issued from a pipe and strikes into at a center of the circular target wall. Mathematical model is based in the solution to RANS equations for the two-phase flow in Euler approximation. For the calculation of the fluctuation characteristics of the dispersed phase equations of Zaichik et al. (1997) [35] model were applied. Predictions were performed for the distances between the nozzle and target plate x/(2R) = 1–10 and the initial droplets size (d₁ = 5–100 μm) at the fixed Reynolds number based on the nozzle diameter, Re = 26,600. Addition of droplets causes significant increase of heat transfer intensity in the vicinity of the jet stagnation point compared with the one-phase air impact jet.     

Thermal-fluid characteristics of plate-fin heat sinks cooled by impingement jet

This work experimentally and numerically studies the thermal-fluid characteristics of plate-fin heat sinks under impingement cooling by adjusting the impinging Reynolds number, the impingement distance, and the fin dimensions. The parameters and the ranges under consideration are the impinging Reynolds number (Re = 5000–25,000), the impingement distance (Y/D = 4–28), the fin width (W/L = 0.08125–0.15625) and the fin height (H/L = 0.375–0.625). The results show that the heat transferred by the heat sink increases with the impinging Reynolds number. The thermal performance can be improved significantly even at low impinging Reynolds number. However, the improvement becomes indistinct as the impinging Reynolds number increases. The thermal resistance declines as the impingement distance increases, and is minimal at Y/D = 20 for various impinging Reynolds numbers. Additionally, the thermal resistance increases as the impingement distance increases further. Increasing the fin width can effectively reduce the thermal resistance. However, as the fin width increases beyond a particular value, the thermal resistance increases dramatically. Reducing the thermal resistance by increasing the fin height depends on a suitable impinging Reynolds number and fin width. Therefore, the effect of the fin height is weaker than that of the impinging Reynolds number or the fin width.     

Numerical simulation of the quenching process in liquid jet impingement

Direct numerical simulation is performed for quenching of a hot plate in liquid jet impingement. The flow and thermal characteristics associated with the quenching process, which includes film boiling in the fluid region as well as transient conduction in the solid region, are investigated by solving the conservation equations of mass, momentum and energy in the liquid, gas and solid phases. The liquid–vapor and liquid–air interfaces are tracked by the sharp-interface level-set method modified to treat the effect of phase change. The computations demonstrate that the boiling curve of wall heat flux versus temperature does not depend on the transient or steady-state heating conditions. The effects of initial solid temperature and solid properties on the quenching characteristics are quantified.     

Boiling curves in relation to quenching of a high temperature moving surface with liquid jet impingement

Experimental investigations have been conducted for quenching of a hot rotating cylinder with initial temperature of about 500–600 °C by a subcooled planar water jet. An original experimental device allowing the estimation of the local boiling curves in the case of a static surface and of a moving surface has been designed. Heat fluxes were measured on both side of the axis of the jet until a reduced distance x/l of 18, in a range of subcooling from 10 to 83 K, for a jet velocity from 0.8 to 1.2 m/s and for a velocity flow-surface ratio (u_S/u_j) from 0.5 to 1.25. In the case of static surface, the measurements confirmed the existence of a “shoulder of flux” in the stagnation zone of the jet. In the case of a moving surface, the maximum of heat transfer (for a given regime) is moving during the cooling time from downstream (film boiling regime) to upstream (forced convection).     

Nucleate boiling on the superhydrophilic surface with a small water impingement jet

In this study, the coating process on the copper surface with titanium dioxide (TiO₂) has been introduced. The coated surface exhibits extremely high affinity for water and the solid–liquid contact angle decreases nearly to zero by exposing the surface to ultra-violet light. This superhydrophilic characteristic was applied to nucleate boiling heat transfer of water jet impingement on a flat heated plate. By making use of this special heat transfer surface, the nucleate boiling heat transfer and the critical heat flux (CHF) of a bar water jet impingement on a large flat superhydrophilic surface was experimentally investigated. The experimental data were measured in a steady state. The purified water was employed as the working liquid. Three main influencing factors, i.e., subcooling, impact velocity and the surface coating condition, were changed and their effects on the nucleate boiling heat transfer and the CHF were investigated. The empirical correlations were obtained for predicting the CHF of steady boiling for a small round water jet impingement on a large flat superhydrophilic surface. The experimental results show that the CHF on the superhydrophilic surface is about 30% higher than that on conventional copper surface by decreasing the solid–liquid contact angle.     

Characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film

An experimental study has been conducted to elucidate characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film. The experiment was done at atmospheric pressure for the following conditions: an initial surface temperature from 200 to 400^°C, a subcooling of 20– 80 K, average velocity of 0.52– 1.24 m/s, and the block material is copper and carbon steel. The surface temperature and heat flux are estimated from the measured temperatures in the block during the quench by a two‐dimensional inverse solution. It follows that as the position of wetting advances downward, the position at which the heat flux becomes a maximum also advances downward. The time at which the position of maximum heat flux begins to move is one of the most important parameters and can be predicted by a proposed correlation. In addition, it is revealed that the maximum heat flux for copper depends on the length to which it occurs from the leading edge. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 345– 360, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20167     

The effect of surface renewal due to largescale eddies on jet impingement heat transfer

The mechanism for the enhancement of stagnation-point heat transfer was explored analyzing the large-scale turbulent structures of an impinging round jet by a statistical technique with conditional sampling. It has been found that large-scale eddies impinging on the heat transfer surfaces produce a turbulent surface-renewal effect dominant for the enhancement of the jet impingement heat transfer. The effect of heat transfer enhancement can be described in terms of the turbulent Reynolds and Strouhal numbers based on the characteristic turbulence intensity and frequency of the large-scale eddies impinging on the stagnation-point boundary layer.     

On the size of the boiling region in jet impingement quenching

Experimental investigations of jet impingement quenching for three different cylindrical blocks made of copper, brass and steel have been conducted with block initial temperature from 250 to 600 °C. Visible observations during the quench show that the wetted area can be divided into two regions – a central region with no apparent boiling and the outer annular region where the liquid boils vigorously. The width of the boiling region is of interest since there is a coupling between high heat transfer rates and the observed boiling pattern. Boiling width increases with material conductivity and decreases with jet subcooling and velocity. Boiling width is also influenced by the initial temperature of the solid.