Jet Inlet Temperature Research Articles

Effect of Strip-Fin Height on Jet Impingement Heat Transfer in a Rectangular Channel at Two Jet-to-Target Surface Spacings

Abstract An experimental investigation of heat transfer performance in a rectangular impingement channel featuring staggered strip-fins was completed. Four configurations were considered to study the effects of varying the strip-fin height (H/d = 1.5 and 2.75) at two jet-to-target surface spacings (z/d = 3 and 6) on the heat transfer, pressure loss, and crossflow magnitude for a long impingement channel with in-line, 4 × 12 impinging jets. Also, the effect of the reference temperature choice, either jet inlet temperature or local bulk temperature, for calculating the local heat transfer coefficients was considered. The regionally averaged heat transfer coefficients were measured at seven Reynolds numbers, based on the jet diameter (10k–70k) utilizing the copper plate experimental method. The empirical correlations were expressed for the area averaged Nusselt number estimation of impingement channels with strip-fin or pin-fin roughness elements. The results showed that the long strip-fin channel with z/d = 3 provided the best thermal performance. The discharge coefficients are similar for all configurations between Rejet = 10k and 50k. The results are compared with the impingement channels with conventional pin-fins. They show that strip-fin channels provide lower pressure drop with marginally better heat transfer coefficients compared to the conventional pin-fin channels. However, when the channel weight is considered, strip-fins would increase the roughness material volume more than the conventional pin-fins.

Numerical Study of Mixed Convection of Buoyant Twin Jet

Abstract The effect of mixed convection of twin vertical jets is investigated numerically in this paper. The results are presented specifically for flows affected by buoyancy for two parallel jets of same velocities ranging between 0.25 m/s and 5.0 m/s and temperatures between 295 K and 320 K. Both jets generate a slow flow with a temperature difference (with the ambient flow) less or equal to 32 °C (305 K). Predictions of dynamical and thermal parameters are obtained for the merging and combining regions. This study reveals that the trajectory of the two jets is strongly influenced by the ratio of buoyancy to inertial forces. Results indicate that, relative to isotherm jets, the location along the vertical symmetry plane at which the two jets merge (merging point) decreases with increasing jet inlet temperature. It was also found that the location of the merging point is shifted toward the confining wall as the velocity of the jets increases. The behavior law (linear regression), relating to the expansion of the jet, is not verified in the developed region for each value of the inlet velocity and temperature. This is explained by the fact that natural convection is more predominant than forced convection. The results show that the self-similarity of the cross profiles of the mean velocity and the behavior law relating to the expansion of the jet are obtained throughout the developed region.

Numerical simulation on jet impingement heat transfer with microcapsule phase change material suspension: Single jet and array jet

The heat transfers on the single and array jet impingement with microcapsule phase change material (MEPCM) suspension are numerically investigated. The equivalent heat capacity method and DPM model are employed to treat the latent heat absorption and motion of the MEPCM particles in suspension, respectively, as well as the VOF method for tracking the free surface from suspension to air. The results show that the latent heat absorption and disturbance effect of the MEPCM particles can significantly improve the heat transfer of the suspension compared to the water. There exists an optimal jet distance at ΔL/D = 4.2 and an optimal suspension concentration at ηp = 5% to achieve the optimal heat transfer. The jet inlet temperature has a favourable range for the heat transfer enhancement, −0.83 <θj < 0.39, which is lower than the phase change temperature range of MEPCM particles. Compared to single jet, the array jet may provide a better heat transfer although the cross-flow between neighboring jets weakens the heat transfer of central jet column. Both of hole number and hole spacing for array nozzle have a non-monotonic effect on the heat transfer, with the optimums at N = 19 and S/D = 7, respectively.

Jet Impingement Cooling Enhanced with Nano-Encapsulated PCM

In the present study, the laminar flow and heat transfer of water jet impingement enhanced with nano-encapsulated phase change material (NEPCM) slurry on a hot plate is analytically investigated for the first time. A similarity solution approach is applied to momentum and energy equations in order to determine the flow velocity and heat transfer fields. The effect of different physical parameters such as jet velocity, Reynolds number, jet inlet temperature, and the NEPCM concentration on the cooling performance of the impinging jet are investigated. The volume fraction of NEPCM particles plays an essential role in the flow and heat transfer fields. The results show that NEPCM slurry can significantly enhance the cooling performance of the system as it improves the latent heat storage capacity of the liquid jet. However, the maximum cooling performance of the system is achieved under an optimum NEPCM concentration (15%). A further increase in NEPCM volume fraction has an unfavorable effect due to increasing the viscosity and reducing the conductivity simultaneously. The effect of adding nano-metal particles on the heat transfer performance is also investigated and compared with NEPCM slurry. NEPCM slurry shows a better result in its maximum performance. Compared with the water jet, adding nano and NEPCM particles would overall enhance the system’s thermal performance by 16% and 7%, respectively.

Experimental study of jet impingement heat transfer with microencapsulated phase change material slurry

Based on the heat dissipation requirement of high heat flux for electronic chips, this paper establishes a closed-cycle experimental system and investigates jet impingement heat transfer characteristics with a microencapsulated phase change material (MEPCM) slurry as the working fluid. The phase change properties of the MEPCM particles and the thermal properties of the MEPCM slurry are analysed. The effects of several key parameters on the heat transfer and pressure drop of jet impingement are also discussed. Experimental results show that a 10% mass fraction of MEPCM slurry may enhance jet heat transfer by 32.8% compared with water because of the latent heat absorption of the PCM core. The heat transfer enhancement will be limited for higher mass fractions of slurry. The slurry exhibits a high heat transfer efficiency when the jet distance is ΔL/D = 4–9.3. The convective heat transfer coefficient of the slurry first increases and then decreases as the jet inlet temperature increases, and the optimum jet inlet temperature is approximately 12.1 °C lower than the melting peak temperature. Jet temperatures that are too high or too low will lead to deterioration of heat transfer for the slurry. These findings can provide guidance for the design of heat dissipation systems using MEPCM slurries.

Study of Conjugate Heat Transfer from Heated Plate by Turbulent Offset Jet in Presence of Freestream Motion using Low-Reynolds Number Modeling

The present study deals with conjugate heat transfer from a heated flat plate by a turbulent offset jet in presence of freestream motion. The turbulent convection in fluid and conduction in solid is solved in a coupled manner by simultaneously satisfying the equality of temperature and heat flux at the solid-fluid interface. The computations have been carried out using low-Reynolds number (LRN) k −ω SST model in the fluid region. The capability of LRN modeling have enabled to solve the entire boundary layer including the thin viscous sublayer due to which Moffatt vortices (secondary recirculation regions) have been captured near the corner of the wall where the turbulence Reynolds number is low. The bottom surface of solid plate is maintained at a constant temperature higher than the jet inlet temperature whereas the jet inlet temperature is same as that of the ambient. The present investigation reports the effects of offset ratio of jet (OR), Reynolds number of flow (Re), solid to fluid thermal conductivity ratio (K), solid slab thickness (S) and freestream velocity (U∞) on conjugate heat transfer arises due to solid and fluid interaction. The offset ratio is varied in the range OR = 3 − 11, Reynolds number in the range Re = 10000 − 25000, solid to fluid thermal conductivity ratio in the range K = 1 − 2000, solid slab thickness in the range S = 1−20 and freestream velocity in the range U∞ = 0.1 − 0.25. The effects of various parameters on the near-wall velocity profile, solid-fluid interface temperature, local Nusselt number variation along the plate, heat flux variation along the plate, etc. have been discussed in detail.

Experimental study of jet impingement cooling with carbon dioxide at supercritical pressures on micro structured surfaces

Jet impingement cooling with carbon dioxide at supercritical pressures on micro pin fin structured surfaces was studied experimentally to enhance the impingement cooling heat transfer. The effects of the microstructure, mass flow rate and heat flux on the heat transfer were studied using the MEMS (Micro-Electro-Mechanical System) based silicon chips for heating and simultaneous surface temperature measurement. The pressure was 7.85 Mpa, which is higher than the critical pressure of carbon dioxide of 7.38 MPa. The jet inlet temperature was 19.0 °C, which is lower than the pseudocritical temperature of 33.78 °C at 7.85 MPa. The mass flow rates were 14.4 kg/h and 20.7 kg/h with the heat flux varying from 0.01 to 0.35 MW/m2. The results showed that the surface area increased by the micro structured surfaces was the major reason for heat transfer enhancement with the circular pin fins giving greater heat transfer enhancement than the square pin fins with the same total surface area. The high specific heat fluid layer near the cooled surface at supercritical pressures enhanced the heat transfer rate with the average heat transfer coefficient first increasing with increasing heat flux and then decreasing with further increases in the heat flux.

Heat transfer rate and uniformity of mist flow jet impingement for glass tempering

The thinning and miniaturization of components, such as liquid crystal display and solar cells, has increased the market demand for tempered ultra-thin glass. The physical tempering in glass, which is less than 2mm thick, is difficult to achieve in air jet impingement. In this study, we propose the use of mist flow jet impingement cooling technology on ultra-thin glass. The effects of the mist flow droplets diameters, nozzle inlet temperature, and mass fraction on the heat transfer rate and uniformity are numerically studied. Our performance in terms of the heat transfer rate and the uniformity is accounted for through an evaluation of the surface-averaged temperature, averaged Nusselt number, surface-averaged Nusselt number, and surface standard deviation percentage of the Nusselt number. The empirical correlation for the surface averaged Nusselt number asa function of mist flow droplet diameters is provided. The droplet diameter is the major factor affecting the heat transfer rate, while the jet inlet temperature is a minor factor. We can obtain a better heat transfer uniformity by controlling the droplet diameter of the mist flow jet impingement. Using mist flow jet impingement, we would not only greatly reduce the amount of air and save energy, but also meet the needs of sudden cooling for tempered glass with a thickness of less than 2mm.

Numerical study on free-surface jet impingement cooling with nanoencapsulated phase-change material slurry and nanofluid

The free-surface jet impingement technique operating with two-phase advanced coolants has recently drawn much favorable attention in cooling applications. However, numerical understanding of free-surface jets along with improved realization of pros and cons associated with these advanced coolants remains a challenge. In this work, the flow and thermal performances of free-surface jet impinging on a heated copper plate are numerically investigated using water, nanoencapsulated phase-change material (NEPCM) slurry, and nanofluid as coolants. Three-dimensional continuity, momentum, and energy equations are discretized with a commercial finite volume code in accordance with a standard k-ε turbulence model. The volume of fluid multiphase technique is adopted in this study to model the free surface between the liquid jet and surrounding ambient air. A single-phase fluid approach is employed using existing models from other references to determine the effective thermophysical properties of NEPCM slurry and nanofluid. The predicted Nu and pressure drop calculations agreed well with the experimental data from references. Physical understanding of the effects of fluid jet inlet temperature, nozzle-to-target distance, and nanoparticle concentration is reported. The addition of both NEPCM and Al2O3 particles to water helps in improving the Nusselt number and decreases the stagnation point temperature with certain pressure drop penalty. The NEPCM slurry enhances the cooling performance of the system by improving its latent heat storage capability, whereas nanofluids improve the cooling performance by enhancing the effective thermal conductivity. The thermal performance can be further improved with increased particle concentration. The NEPCM slurry demonstrates performance superior to nanofluid working at the same particle loading. Overall, the proposed model can provide valuable guidelines for the use of advanced coolants in a free-surface jet impingement cooling system.

Experimental and Numerical Heat Transfer Investigation of an Impinging Jet Array on a Target Plate Roughened by Cubic Micro Pin Fins1

A generic impingement cooling system for turbomachinery application is modeled experimentally and numerically to investigate heat transfer and pressure loss characteristics. The experimental setup consists of an array of 9 × 9 jets impinging on a target plate with cubic micro pin fins. The cubic micro pin fins have an edge length of 0.22 D and enlarge the target area by 150%. Experimentally heat transfer is measured by the transient liquid crystal (TLC) method. The transient method used requires a heated jet impinging on a cold target plate. As reference temperature for the heat transfer coefficient, we use the total jet inlet temperature which is measured via thermocouples in the jet center. The computational fluid dynamics (CFD) model was realized within the software package ANSYS CFX. This model uses a Steady-state 3D Reynolds-averaged Navier–Stokes (RANS) approach and the shear stress transport (SST) turbulence model. Boundary conditions are chosen to mimic the experiments as close as possible. The effects of different jet-to-plate spacing (H/D = 3–5), crossflow schemes, and jet Reynolds number (15,000–35,000) are investigated experimentally and numerically. The results include local Nusselt numbers as well as area and line averaged values. Numerical simulations allow a detailed insight into the fluid mechanics of the problem and complement experimental measurements. A good overall agreement of experimental and numerical behavior for all investigated cases could be reached. Depending on the crossflow scheme, the cubic micro pin fin setup increases the heat flux to about 134–142% compared to a flat target plate. At the same time, the Nusselt number slightly decreases. The micro pin fins increase the pressure loss by not more than 14%. The results show that the numerical model predicts the heat transfer characteristics of the cubic micro pin fins in a satisfactory way.

Numerical analysis of turbulent round jet impingement heat transfer at high temperature difference

Heat transfer of round jet impingement at high temperature difference was numerically investigated with V2F turbulent model. Validation of the model was evaluated against available experimental data. Effect of the density and thermal properties on heat transfer was studied. Numerical results show that the decrease of density leads to decrease of the heat transfer coefficient while the increase of each thermal property leads to increase of the heat transfer coefficient. Meanwhile, Nusselt number is found to be independent of the temperature difference and can be calculated from the correlation at low temperature difference on the condition that the jet inlet temperature is selected as the qualitative temperature. Correlation proposed by earlier researcher is then modified for impingement heat transfer at high temperature difference.

Modeling Lifted Jet Flames in a Heated Coflow Using an Optimized Eddy Dissipation Concept Model

Moderate or intense low oxygen dilution (MILD) combustion has been established as a combustion regime with improved thermal efficiency and decreased pollutant emissions, including NOx and soot. MILD combustion has been the subject of numerous experimental studies, and presents a challenge for computational modeling due to the strong turbulence–chemistry coupling within the homogeneous reaction zone. Models of flames in the jet in hot coflow (JHC) burner have typically had limited success using the eddy dissipation concept (EDC) combustion model, which incorporates finite-rate kinetics at low computational expense. A modified EDC model is presented, which successfully simulates an ethylene-nitrogen flame in a 9% O2 coflow. It is found by means of a systematic study in which adjusting the parameters and from the default 0.4082 and 2.1377 to 3.0 and 1.0 gives significantly improved performance of the EDC model under these conditions. This modified EDC model has subsequently been applied to other ethylene- and methane-based fuel jets in a range of coflow oxidant stream conditions. The modified EDC offers results comparable to the more sophisticated, and computationally expensive, transport probability density function (PDF) approach. The optimized EDC models give better agreement with experimental measurements of temperature, hydroxyl (OH), and formaldehyde (CH2O) profiles. The visual boundary of a chosen flame is subsequently defined using a kinetic mechanism for OH* and CH*, showing good agreement with experimental observations. This model also appears more robust to variations in the fuel jet inlet temperature and turbulence intensity than the standard EDC model trialed in previous studies. The sensitivity of the newly modified model to the chemical composition of the heated coflow boundary also demonstrates robustness and qualitative agreement with previous works. The presented modified EDC model offers improved agreement with experimental data profiles than has been achieved previously, and offers a viable alternative to significantly more computationally expensive modeling methods for lifted flames in a heated and vitiated coflow. Finally, the visually lifted flame behavior observed experimentally in this configuration is replicated, a phenomenon that has not been successfully reproduced using the EDC model in the past.

Conjugate heat transfer study of incompressible turbulent offset jet flows

In the present case, the conjugate heat transfer involving a turbulent plane offset jet is considered. The bottom wall of the solid block is maintained at an isothermal temperature higher than the jet inlet temperature. The parameters considered are the offset ratio (OR), the conductivity ratio (K), the solid slab thickness (S) and the Prandtl number (Pr). The Reynolds number considered is 15,000 because the flow becomes fully turbulent and then it becomes independent of the Reynolds number. The ranges of parameters considered are: OR = 3, 7 and 11, K = 1–1,000, S = 1–10 and Pr = 0.01–100. High Reynolds number two-equation model (k–e) has been used for turbulence modeling. Results for the solid–fluid interface temperature, local Nusselt number, local heat flux, average Nusselt number and average heat transfer have been presented and discussed.

Modeling Study on the Entrainment of Ambient Air into Subsonic Laminar and Turbulent Argon Plasma Jets

Modeling study is performed to reveal the special features of the entrainment of ambient air into subsonic laminar and turbulent argon plasma jets. Two different types of jet flows are considered, i.e., the argon plasma jet is impinging normally upon a flat substrate located in atmospheric air surroundings or is freely issuing into the ambient air. It is found that the existence of the substrate not only changes the plasma temperature, velocity and species concentration distributions in the near-substrate region, but also significantly enhances the mass flow rate of the ambient air entrained into the jet due to the additional contribution to the gas entrainment of the wall jet formed along the substrate surface. The fraction of the additional entrainment of the wall jet in the total entrained-air flow rate is especially high for the laminar impinging plasma jet and for the case with shorter substrate standoff distances. Similarly to the case of cold-gas free jets, the maximum mass flow-rate of ambient gas entrained into the turbulent impinging or free plasma jet is approximately directly proportional to the mass flow rate at the jet inlet. The maximum mass flow-rate of ambient gas entrained into the laminar impinging plasma jet slightly increases with increasing jet-inlet velocity but decreases with increasing jet-inlet temperature.

Numerical study of flow and thermal oscillations in buoyant twin jets

Non-isothermal twin parallel jets in horizontal orientation, are studied numerically to ascertain the mean flow structure and the oscillation characteristics of temperature and velocity fields. The analysis is carried out for Reynolds number ranging between 9000 and 12,000 and Grashof number between 50 and 1000. The simulated results compare well with available experimental data on axial velocity distribution and jet merger distance. Subsequently, a parametric study is presented to bring out the effects of Reynolds number, nozzle spacing and jet inlet temperature.

A Numerical Study of Buoyant Plane Parallel Jets

A numerical study was performed to assess the influence of buoyancy on plane, parallel jets. Results indicate that, relative to isothermal jets, the location along the vertical symmetry plane at which the two jets merge (the merge point) decreases with increasing jet inlet temperature. This decrease is attributed to higher entrainment rates for the heated jet relative to the isothermal jet. It was also found that for sufficiently high values of the Archimedes number, the merge point becomes nearly independent of the initial jet spacing.

Effect of liquid temperature on spray flash evaporation

To complement the authors' previous experimental study of spray flash evaporation pursued at 60°C jet inlet temperature, an additional experimental study has been made of the effect of liquid temperature on the spray flash evaporation by changing jet inlet temperatures to 40 and 80°C. From the experimental results, a more general empirical equation suitable for predicting the variation of liquid temperature in the center of jet with residence time was deduced, and the recommendable operating conditions were proposed. Furthermore, an expedient method for prediction of the liquid temperature drop along the length of jet was devised for the recommendable operating conditions. It was realized that, even at lower liquid temperatures, the spray flash evaporation still has higher evaporation performance and extremely faster evaporation rate than the flash evaporation occurring in other systems.

An experimental study of spray flash evaporation

An experimental study has been conducted on spray flash evaporation occurring in a superheated water jet injected through a circular tube nozzle into a low-pressure vapor zone. The effects of superheat, flow rate and nozzle diameter on spray flashing were pursued at 60°C jet inlet temperature. From the experimental results, an empirical equation suitable for predicting the variation of liquid temperature with residence time was deduced.