Jet Reynolds Number Research Articles

Flow structure and surface heat transfer from numerical predictions for a double wall effusion plate with impingement jet array cooling

To provide additional understanding of double wall cooling arrangements, especially local distributions of flow properties which are responsible for hot-side surface and cold-side surface heat transfer variations, investigated are numerically-simulated distributions of turbulent flow structural characteristics. Also considered are numerically-simulated surface heat transfer characteristics, including comparisons with experimentally-measured distributions. The numerical results are obtained using the ANSYS FLUENT Version 19.1 numerical code, with a k-ω SST turbulence model. The present arrangement includes a full-coverage effusion cooling plate, with coolant initially supplied by an impingement jet array. Considered are the effects of effusion blowing ratio, impingement jet Reynolds number, and streamwise development on flow structure, and on hot-side and cold-side surface heat transfer characteristics. Data are obtained for an approximately constant main flow Reynolds number of 138000 to 159000, with blowing ratios from 2.2 to 8.5, which correspond to impingement jet Reynolds numbers from 7360 to 25200. Of particular importance are horseshoe-shaped vortices, which form upstream and around each effusion jet, which then result in two horseshoe-originated vortex legs which advect downstream along coolant trajectories. Also important are secondary flows, local flow separation regions, and local velocity regions, which are evident within the effusion hole passages near entrance locations. Within the cross flow passage, locally increased flow velocities, associated with impingement jet locations, are apparent, especially near the lower surface of the cross flow passage. These are associated with regions of augmented streamwise vorticity, which are present around the circumference of each impingement jet.

Experimental Investigation on the Heat Flux Distribution and Pollutant Emissions of Slot LPG/Air Premixed Impinging Flame Array

Experiments were carried out to investigate the heat transfer and pollutants emission characteristics of a slot LPG premixed flame array impinging normally onto a flat plate. The effects of jet-to-jet spacing (S/de), nozzle-to-plate distance (H/de), and jet Reynolds number (Re) on the heat flux and emission index of CO, CO2, and NOx/NO2 were examined. In addition, the thermal and emission characteristics between slot jets and circular jets were compared under identical experimental conditions. The results show that the more uniform heat flux distribution and higher total heat flux can be obtained at moderate jet-to-jet spacing, large jet-to-plate distance, and higher Reynolds number. EICO emissions can be influenced by the combined effects of jet-to-jet spacing, jet-to-plate distance, and higher Reynolds number. For the sake of the better combustion efficiency and lower EICO emission, the moderate jet-to-jet spacing (S/de = 2.5), larger jet-to-plate distance (H/de = 4), and relatively higher Reynolds number (Re = 1500) are preferred for the slot jet flame array. Furthermore, it is found that there exists a trade-off between the EICO and EINOx of the slot LPG flame array. Compared with multiple circular flame jets, multiple slot flames jets have the higher area-averaged heat flux due to the larger heating area and more uniform heat flux distribution, while the higher EICO emission and lower EINOx emission are due to the greater jet interaction suppressing the air entrainment. Thus, it is known that the slot flame array has a better heating performance but relatively higher pollutant emissions than the circular flame array.

Effect of co-flow on fluid dynamics of a cough jet with implications in spread of COVID-19.

We discuss the temporal evolution of a cough jet of an infected subject in the context of the spread of COVID-19. Computations were carried out using large eddy simulation, and, in particular, the effect of the co-flow (5% and 10% of maximum cough velocity) on the evolution of the jet was quantified. The Reynolds number (Re) of the cough jet, based on the mouth opening diameter (D) and the average cough velocity, is 13 002. The time-varying inlet velocity profile of the cough jet is represented as a combination of gamma-probability-distribution functions. Simulations reveal the detailed structure of cough jet with and without a co-flow for the first time, to the best of our knowledge. The cough jet temporal evolution is similar to that of a continuous free-jet and follows the same routes of instability, as documented for a free-jet. The convection velocity of the cough jet decays with time and distance, following a power-law variation. The cough jet is observed to travel a distance of approximately 1.1 m in half a second. However, in the presence of 10% co-flow, the cough jet travels faster and covers the similar distance in just 0.33 s. Therefore, in the presence of a co-flow, the probability of transmission of COVID-19 by airborne droplets and droplet nuclei increases, since they can travel a larger distance. The cough jet without the co-flow corresponds to a larger volume content compared to that with the co-flow and spreads more within the same range of distance. These simulations are significant as they help to reveal the intricate structure of the cough jet and show that the presence of a co-flow can significantly augment the risk of infection of COVID-19.

Experimental Study of Impingement Effusion Cooled Double-Wall Combustor Liners: Aerodynamic Analysis with Stereo-PIV

A new experimental study is presented for a combustor with a double-wall cooling design. The inner wall at the hot gas side features effusion cooling with 7-7-7 laidback fan-shaped holes, and the outer wall at the cold side features an impingement hole pattern with circular holes. Data are acquired to asses the thermal and aerodynamic behavior of the setup, using a new, scaled up, engine similar test rig. Similarity includes Reynolds, Nusselt and Biot numbers for hot gas and coolant flow. Different geometrical setups are studied by varying the cavity height between the two walls and the relative alignment of the two hole patterns at two different impingement Reynolds numbers. This article focuses on the aerodynamic performance of the setup. Instationary flow data are acquired, using a high speed stereo PIV setup. For each geometrical configuration, approximately 20 planes are recorded with a data rate of 1000 Hz by traversing the flow region of interest in the cavity between the two specimen. This fine resolution allows the reconstruction of 3D flow fields for the mean data values and an extensive analysis of transient phenomena at each plane. Time averaged data and jet-center plane transient data are presented in detail. The results show a complex flow field with a hexagonal vortex pattern in the cavity, which is mainly influenced by the cavity height and the relative alignment of the two walls. The jet Reynolds number shows small influence when analyzing normalized data. Small cavity heights show a less developed flow field with less stable vortex systems. The alignment shows a similar influence on vortex system stability, with the aligned case performing better. Additionally, statistical analysis of the jet flow and frequency domain analysis of the jet and the effusion flow are presented, showing the damping capability of the cavity, especially at increased cavity heights, and a residual low frequency pulsation of the effusion cooling inflow.

Flow and heat transfer characteristics in a pre-swirl rotor-stator cavity

The present study investigates flow and heat transfer characteristics in a rotor-stator pre-swirl system. Considered are rotational Reynolds numbers ReФ ranging from 3.4 × 105 to 6.8 × 105, with jet Reynolds numbers Rew of approximately 8.8 × 104 to 3.81 × 105, and associated turbulent flow parameter λT = Cw/ReФ0.8 varying from 0 to 0.4. Experimentally-measured and numerically-predicted results illustrate the combined and separate influences of jet impingement and rotor entrainment, as they affect and govern flow and heat transfer characteristics within the cavity. These results show that the relative strength of the two effects varies with ReФ and Rew, with similar performance for flow conditions with similar magnitudes of the turbulent flow parameter λT. When 0<λT < 0.27, jet impingement and rotor entrainment both have important influences on flow and heat transfer characteristics. However, when λT > 0.27, the effects of rotor entrainment are less pronounced.

Heat transfer enhancement of impingement cooling with corrugated target surface

Impingement cooling is an important turbine blades cooling technique, but it is often not as efficient as required due to the deflection of downstream jets caused by crossflow. In this paper, a novel corrugated target surface is proposed to convert downstream oblique impingement into orthogonal impingement for higher heat transfer. To prove this design, the flow and heat transfer characteristics of impingement cooling with conventional flat target surface (Baseline case), full-corrugated (FC) target surface, and semi-corrugated (SC) target surface are compared over the jet Reynolds number ranging from 15,000 to 45,000. Results show that the Nusselt number of full-corrugated target surface is almost the same as that of flat target surface at the region corresponding to the first five jets, but its peak value and uniformity are significantly improved at the region corresponding to the last four jets, especially under high jet Reynolds numbers. In addition, with the increase of corrugation depth (H), the total heat transfer capacity and area-averaged Nusselt number increase, but this comes at the cost of increased friction loss. Comprehensively evaluating heat transfer and friction loss, only the FC with H = 0.8D case always has better thermal performance than Baseline case under all computed conditions, and the maximum improvement can reach 5.8%. Last but not least, the SC with H = 0.8D case is proposed based on the FC with H = 0.8D case to further reduce the friction loss, and results show that its thermal performance is improved significantly due to the decreased friction loss and almost unchanged heat transfer.

Dynamics of circular and rectangular jets in crossflow

In this work, jets in crossflow (JICFs) were studied numerically through large-eddy simulation using a velocity ratio of 5.7 and a jet Reynolds number of 5000. A parabolic profile was used for the jet inlet and four specific cases were compared: one with a circular inlet and three with rectangular pipe inlets (aspect ratios: 2:1, 0.5:1, and 3:1) having the same mass flow rate of the jet. The results indicated that the rectangular JICFs penetrated the domain more deeply than the circular JICF and delayed the vortex and turbulence breakdown. The effect of the crossflow on the jet orientation was also studied and quantified, and modal analysis was conducted on the cases with dynamic mode decomposition. The significant modes were found to be consistent with spectral analysis of convectively unstable JICFs in the literature. The rectangular JICFs showed increased dominant modes frequencies compared to the circular JICF.

Experimental Investigation of Micro Cooling Units on Impingement Jet Array Flow Pressure Loss and Heat Transfer Characteristics

With the development in additive manufacturing, the use of surface treatments for gas turbine design applications has greatly expanded. An experimental investigation of the pressure loss and heat transfer characteristics within impingement jet arrays with arrays of target surface micro cooling units is presented. The discharge coefficient and Nusselt number are measured and determined for an evaluation of the pressure loss of the flow system and heat transfer level, respectively. Considered are effects of impingement jet Reynolds number ranging from 1000 to 15,000 and micro cooling units (square pin fin) height (h) with associated values of 0.01, 0.02, 0.05, 0.2, and 0.4 D, where D is the impingement hole diameter. Presented are variations of Nusselt number, and Nusselt number ratio, discharge coefficient, discharge coefficient ratio, discharge coefficient correlation. Depending upon the micro cooling unit height, discharge coefficient ratios slightly decrease with height, and the ratio values generally remain unit value (1.0). When Rej = 1000 and 2500 for several cooling units height values, discharge coefficient ratios show the pressure loss decreases about 2–18% and 3–6%, respectively, when compared to the data of a baseline smooth target surface plate. The observed phenomenon is due to the effects of flow blockage of micro cooing units, local flow separation, and near-wall viscous sublayer reattachment. Results also show that heat transfer levels increase 20–300% for some of the tested toughened target surface plates when compared to smooth target surface plates. The heat transfer level enhancement is because of an increase in thermal transport and near-wall mixing, as well as the increased wetted area. In addition, micro cooling units elements break the viscous sublayer and cause greater turbulence intensity when compared to the smooth target surface. Overall, results demonstrate that the target surface micro cooling units do not result in a visible increment in pressure loss and reduce pressure loss of the flow system for some of the tested patterns. Moreover, results show the significant ability of micro cooling units to enhance the surface heat transfer capability of impingement cooling relative to smooth target surfaces.

Flow and heat transfer in a rotating channel with impingement cooling and film extraction

Impingement cooling is applied on the turbine blade leading edge which suffers the highest heat transfer and needs priority protection. The current work focuses on the rotational effects in an impingement cooling channel with film extraction, in which heat transfer is obtained with experiment whereas the flow field is predicted by numerical simulation. The dimensionless spacing of jet-to-jet (s/Dj) and jet-to-target surface (l/Dj) are both 3. The jet Reynolds number and the channel orientation (the angle between jet direction and rotating orientation) are 5,000 and 135°, respectively. The jet rotation number changes from 0 to 0.24, and the maximum jet buoyancy number reaches 0.57. The results show that the heat transfer on the suction side is better than that on the pressure side where the recirculation region generates. The heat transfer deteriorates from high to low radius due to the non-uniform mass flow rate distribution. Once the channel rotates, the non-uniformity of mass flow rate increases because the vortex occurs in the supply channel, resulting in the heat transfer increase by 140% in the low radius region on the pressure side. On the pressure side in the impingement channel, the rotation-induced secondary flow breaks the recirculation region, promoting the heat transfer. On the suction side, however, the secondary flow and rotation-driven jet deflection decrease the heat transfer. Besides, the jet buoyancy number is inappropriate to describe the combined influences of jet rotation number and wall-to-fluid temperature ratio at high jet buoyancy number in current experimental conditions.

Vortex breakdown mechanics of a laminar confined swirling flow with large expansion ratio using a non-uniform finite difference approximation

Abrupt expansions are a very frequent geometry in mechanical engineering systems, i.e. in combustion chambers, valves, heat exchangers or impinging cooling devices. However, despite the large number of devices that use this geometry, the expanded flow behaviour still needs further research to understand and predict the full system performance. This paper presents the application of the non-uniform finite difference approximation method developed in Sanmiguel et al. for the numerical characterisation of a confined swirling laminar jet discharging with a large expansion ratio. This investigation can be considered an extension of previous work by Revuelta, but now a swirling flow is generated by a rotating pipe upstream the expansion. The structures found when a fully-developed rotating Hagen-Poiseuille flow discharges into a much larger pipe section are summarised in a bifurcation diagram, whose coordinates are the Reynolds number of the jet ( Rej) and the swirl parameter ( L), for which the time-dependent, axisymmetric and incompressible Navier-Stokes equations are integrated numerically. For values of the jet Reynolds number below 200, there is a critical value of the swirl parameter above which stable vortex breakdown appears. For values of the Reynolds number above 200, three different behaviours are observed, and each performance appears for a critical value of the swirl parameter. When increasing the swirl parameter from zero, the flow becomes axisymmetrically unstable, showing an oscillatory behaviour. If further increasing the swirl intensity, the oscillatory flow coexists with a vortex breakdown bubble and, finally, a steady vortex breakdown is reached. The expansion ratio ε considered in all the simulations is 1[Formula: see text]. In previous literature, the exactness of the limiting critical Rej and L values that define these behaviours has been found to be influenced by the variability in the inlet profile conditions, which affects the expanded flow. This enhances the importance in the present investigation to accurately simulate the discharge pipe inlet profiles.

Investigations of Flow and Heat Transfer Characteristics in a Channel Impingement Cooling Configuration with a Single Row of Water Jets

The present study experimentally and numerically investigates the effect of channel height on the flow and heat transfer characteristics of a channel impingement cooling configuration for various jet Reynolds numbers in the range of 2000–8600. A single array consisting of eleven jets with 0.8 mm diameter injects water into the channel with 2 mm width at four different channel heights (3, 4, 5, and 6 mm). The average heat transfer coefficients at the target surface are measured by maintaining a temperature difference between the jet exit and the target surface in the range of 15–17 °C for each channel height. The experimental results show the average heat transfer coefficient at the target surface increases with the jet Reynolds number and decreases with the channel height. An average Nusselt number correlation is developed based on 85 experimentally measured data points with a mean absolute error of less than 4.31%. The numerical simulation accurately predicts the overall heat transfer rate within 10% error. The numerical results are analyzed to investigate the flow structure and its effect on the local heat transfer characteristics. The present study advances the primary understanding of the flow and heat transfer characteristics of the channel impingement cooling configuration with liquid jets.

On heat transfer and flow characteristics of jets impingement on a concave surface with varying pin-fin arrangements

Numerical simulations have been conducted to investigate the effects of pin-fin arrangement and jet Reynolds number on heat transfer enhancement and flow characteristics of a concave confined jet impingement cooled surface conjugated with pin-fins. The results without a pin-fin are used as a baseline. Vortices and different sections of streamlines in the channel are employed to understand the flow structures in this cooling system. Based on this, topology pictures are obtained with skin-friction lines to discuss the fluid flow near the target and pin-fin surface. The local and averaged heat transfer characteristics are analyzed using the flow characteristics. The overall parameters (friction factor, thermal performance, pumping power) are also obtained. The results show that the appearance of the pin-fin plays a positive role in heat transfer enhancement and uniformity. The case of a 35° pin-fin arrangement angle reaches the highest overall Nusselt number and heat transfer uniformity. The far away movement of the pin-fin is beneficial to decrease pumping power at a constant heat transfer. The thermal performance is highest for 55° pin-fin arrangement angle at a jet Reynolds number of 40,000 in this work.

Random packing material in disinfection contactors: Effects on baffling and energy loss

AbstractThis study investigates the potential improvement in the hydraulic disinfection efficiency and the possible energy loss as a result of the presence of random packing material (RPM) in a laboratory‐scale chlorine contactor. Tracer tests were conducted on a 55‐gal drum tank filled with RPM in varying amounts in different configurations to measure the efficiency of each setup in terms of baffling factor (BF). The bulk pressure drop was measured to determine the energy loss for each configuration. The results of this study show that securing RPM near the inlet, in any amount, improves the BF by 300% to more than 900%. The amount of RPM begins to have an impact at or above an inlet jet Reynolds number of ~27,700. Also, changes in head loss due to the presence of RPM (in any amount, configuration, and/or flow rate) were generally considered to be negligible.Article Impact StatementThis study highlights the practical utility of random packing material for enhancing baffling factors of contactors in small water systems.

Passive mechanical heat/mass exchange enhancement by semi-confined laminar parallel wall jets

Many different approaches have been investigated in the literature to achieve efficient heat and mass exchangers in applications in several fields such as mechanical, aeronautical or biomedical engineering. Some of these engineering devices need to efficiently mix two fluids at different temperature or concentrations, achieving a decent mixing at a low power cost, and using passive mechanical elements if possible. This paper introduces and analyses numerically the novel application of two parallel wall jets to enhance mixing by vortex shedding induced mechanically. The numerical approach is the non-uniform finite difference approximation, used for the first time in the study of mixing mechanics. The parallel wall jets are constrained within a channel, which can be a simple and passive easily manufactured machine with high potential as scalable laminar mixing device. Different wall jet geometries and Reynolds numbers have been tested with a verified&validated CFD code based on non-uniform finite difference approximation. This analysis allowed to predict configurations that lead to unsteady oscillatory motion. In general, it has been observed that for a range of laminar wall jets, a small channel ratio seems a very efficient option for heat/mass transfer, thanks to the intense oscillation generated in the flow downstream, which enhances mixing with low power requirement.

Experimental and Numerical Heat Transfer Investigation of Impingement Jet Nozzle Position in Concave Double-Wall Cooling Structures

In this paper, experimental and numerical study has been carried out to investigate impingement cooling with a row of five circular jets, varied between target positions on a realistic leading edge region of gas turbine blade geometry. Experimental data is collected from a transient thermochromic liquid crystal measurement technique at the target surface. Numerical study was conducted with the geometry using commercial computational fluid dynamics software to analyze the fluid flow. The unique aims of the study are to observe the effects of variation in jet location, and those specific to realistic target and nozzle geometries. Distributions of local and average Nusselt number show that a location targeting the concave surface at 90° demonstrates an overall higher heat transfer coefficient, especially in the stagnation region, and toward the airfoil sides, with significantly fewer swirls. The experiment was performed with the following parameters: distance from nozzle to target of 1.7 to 2.1 jet diameters, pitch between jets of 4.4 jet diameters, and concave target diameter of 8.0 jet diameters. The jet Reynolds number range during this test was 20,000 − 40,000. A standard flat target plate impingement test is also experimentally conducted and compared against existing literature for method validation.

Numerical investigation of conjugate heat transfer on a rotating disk under round liquid jet impingement

The two-phase flow and heat transfer characteristics of a free liquid jet impinging on a heated rotating disk are investigated by the CFD method. Numerical simulations are conducted over a range of rotational Reynolds numbers, jet Reynolds numbers, jet temperatures, nozzle diameter to disk spacing ratios, and disk to nozzle diameter ratios. The results show that the local Nusselt number increases first and then decreases along the radial direction, and it reaches the peak value in the core heat transfer zone close to the rotation center. The Nusselt number of the disk surface increases with the increase of rotational Reynolds number, jet Reynolds number, jet temperature, and nozzle diameter to disk spacing ratio, and the temperature distribution of the disk surface becomes more uniform. The increase of the disk to nozzle diameter ratio is not conducive to convective heat transfer. The temperature at the disk edge rises with the larger diameter ratio. Based on the numerical simulation results, a new correlation for the average Nusselt number is presented in terms of the dimensionless variables of jet Reynolds number, rotational Reynolds number, and Prandtl number in this study.

Correlations for heat transfer characteristics of the onset of nucleate boiling in submerged jet impingement

The submerged jet impingement boiling is one of the most effective heat transfer methods in dissipating both transient and steady concentrated heat loads. However, studies on the submerged jet impingement boiling still lack experimental data, especially for the heat transfer characteristics of the onset of nucleate boiling (ONB). Therefore, an experimental study of a submerged circular jet impinging on a polished copper surface is conducted in the subcooled water. The experiments investigate the steady-state heat transfer characteristics at the stagnation point from single phase convection to partial nucleate boiling. The effects of jet Reynolds number, liquid subcooling, and nozzle diameter on the onset of nucleate boiling are systematically studied. The experimental results show that the single phase convective heat transfer can be enhanced by increasing jet velocity at a fixed nozzle diameter or decreasing nozzle diameter under the same jet Reynolds number. The liquid subcooling seems to have negligible effect on the single phase heat transfer coefficient. For the ONB point, it is found that the ONB wall superheat is not influenced by liquid subcooling or jet parameters. The ONB heat flux, on the other hand, is found to depend on the experimental parameters. Increasing the jet Reynolds number and liquid subcooling, or decreasing the nozzle diameter could broaden the single phase convective regime and delay the onset of nucleate boiling, in terms of heat flux. Empirical correlations are proposed for predicting the stagnation point heat transfer coefficient, heat flux, and wall superheat at the ONB point. The comparison between the predicted values and the experimental data indicates that the empirical correlations are able to predict the heat transfer characteristics of the onset of nucleate boiling within an acceptable accuracy under the applicable range.

Experimental Study on Propagation of Particle-Bearing Jets in a Confined Geometry

Abstract The spreading characteristics of particle-bearing jets down a gentle slope in a uniform ambient with confined side-walls are reported through a series of laboratory experiments. A round water jet is mixed with solid particles having particle volume fraction ranging between 0% and 0.4%. The jet Reynolds number is varied between Re = 3000 and 8000. We document that the jet front position varies with time t as xf∝t2/3. The jet front propagation in the presence of side-walls is found to be higher than the unconfined case and is attributed to the recirculation due to the confinement that increases the flow inertia and accelerates the flow. The jet propagation is found to be self-similar and is unaffected by the variations in the volume fraction and Re. The sediment pattern near the source, formed by the settling of the particles, exhibits a similar tear-drop shape, which is well-predicted using the unconfined jet theory that assumes a Gaussian velocity profile. The results have implication in engineering and environmental flows, where higher jet propagation rate for confined jets should be modeled accurately without modifying the sedimentation dynamics.

Multi-spatio-temporal scales PIV in a turbulent buoyant jet discharging in a linearly stratified environment

Time-resolved particle image velocimetry is implemented with a camera array and several laser sheets; this results in a multi-spatio-temporal scale system that is modular and flexible. The setup is optimized to capture global flow features, while locally resolving in space and time near the Kolmogorov scale. The apparatus relies extensively on machine vision CMOS cameras; they are inexpensive and stream to computer hard drive with virtually continuous time-resolved records (up to one hour for the current system). This leads to statistically converged data and also helps in minimizing down time between experimental runs. Details of the implementation and design of experiment are reported.The system instruments a vertical buoyant jet discharging in a linearly stratified environment. Refractive index matched aqueous solutions enable precise optical deployment. The density difference is 3% and the fluids have similar dynamic viscosity. The jet Reynolds number is 2.00×104 and is above the mixing transition. Such flows are typically challenging to instrument and few velocity field data are available to date. Velocity statistics are reported as well as first insights gained from the campaign.

Heat transfer characteristics of an impinging swirling inverse diffusion butane/air flame jet

This paper presents an experimental study to explore the feasibility of utilizing a swirling inverse diffusion flame (IDF) jet for impingement heating. The investigation concentrates on the impinging flame structure and impingement heat transfer. The effects of air swirling, the air jet Reynolds number, Rea, the global equivalence ratio, Φ, and the non-dimensional burner-to-plate distance, H/da, on the impinging flame shape and impingement heat transfer are investigated experimentally. The Experiments show that introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. Under-swirling produces no evident heat transfer enhancement due to insufficient air/fuel mixing, while over-swirling reduces heat transfer rate because of the dilution of the entrained cold ambient air. Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone around a stoichiometric condition. The investigation shows that the impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets with blue appearance, ultra-low NOx emission, high flame temperature, good self-stabilization capability, and enhanced heat transfer rate. Therefore, it is a desirable option for impingement heating applications.