Jet impingement cooling using fluidic oscillators: an experimental study

A growing portion of the thermal load on board airplanes is due to densely packed electronic systems. This increased thermal load along with constraints on weight and volume have made simple and reliable cooling solutions an urgent need in the aerospace industry. There is a wealth of cooling solutions available in order to meet these demands, the simplest and most adaptable of which is probably jet impingement cooling. In this study, fluidic oscillators capable of producing pulsating jets were used to cool a heated surface and were then compared to equivalent steady jets. Although pulsating jets can be produced using a number of devices, fluidic oscillators offer the advantage of not having any moving parts. These oscillators are sustained by a self-induced internal flow instability and can function at different scales. Although the major part of this work is based on prototypes that produce jets with sub-millimetric widths, designs at one tenth that scale, i.e. with an exit slot width of 50 µm, are also presented. Reynolds numbers ranging from ReD = 3500 to 5250 and jet-to-plate spacing from 1D to 10D were studied (where D is the initial width of the jet). The Nusselt number distribution is found for each case and a comparison is made between the performance of equivalent steady and pulsating jets based on the average Nusselt number.

Laminar Free Convection From a Heated Square Cylinder in Bingham Plastic Fluids

The free convection phenomenon from a heated square cylinder submerged in Bingham Plastic fluids is numerically investigated. The governing equations are solved for a wide range of physical dimensionless parameters, such as Rayleigh number (10^2 ≤ Ra ≤ 10^5), Prandtl number (10 ≤ Pr ≤ 100) and Bingham number (0 ≤ Bn ≤ 10^7). The heat transfer characteristics are investigated in terms of local Nusselt number distribution over the surface of the cylinder surface average Nusselt number. Streamlines, isothermal contours, yielded and unyielded regions are visualized in detail.

Modeling of mixed convection in an enclosure using multiple regression, artificial neural network, and adaptive neuro-fuzzy interface system models

In this study, the heat transfer characteristics of laminar combined forced convection through a horizontal duct are obtained with the help of the numerical methods. The effect of the geometrical parameters of the cavity and Reynolds number on the heat transfer is investigated. New heat transfer correlation for hydrodynamically fully developed, laminar combined forced convection through a horizontal duct is proposed with an average error of 6.98% and R2 of 0.8625. The obtained correlation results are compared with the artificial neural network and adaptive neuro-fuzzy interface system models. Due to the obtained results, good agreement is identified between the numerical results and predicted adaptive neuro-fuzzy interface system results. In conclusion, it is seen that adaptive neuro-fuzzy interface system can predict the Nusselt number distribution with a higher accuracy than the developed correlation and the artificial neural network model. The developed adaptive neuro-fuzzy interface system model predicts the Nusselt number with 1.07% mean average percentage error and 0.9983 R2 value. The effect of the different training algorithms and their ability to predict Nusselt number distribution are examined. According to the results, the Bayesian regulation algorithm gives the best approach with a 2.235% error. According to the examination that is performed in this study, the adaptive neuro-fuzzy interface system is a powerful, robust tool that can be used with confidence for predicting the thermal performance.

Evaluating the Effect of Number of Spans on Heat Transfer in Greenhouses

The present study concerns convective flows in the empty volume above the plant canopy in a confined greenhouse. The purpose of this paper is to numerically investigate the effect of the number of spans on the convective heat transfer in closed greenhouses. The initial greenhouse CFD model cavity is validated against experimental results found in the literature. Thermal convection is induced by heating the bottom of the cavity. The numerical model is then modified to represent two-l greenhouse cavities with different numbers of spans. The computational fluid dynamic (CFD) software is then used to analyze mainly the natural convective heat transfer, velocity and temperature distributions for the single span greenhouse, as well as multi-span greenhouses (containing two and three spans). The greenhouse CFD model floor is heated, and the walls are adiabatic, corresponding to Rayleigh-Bénard convection. A mesh sensitivity analysis was conducted to determine a suitable size for the mesh. Results show that adding additional spans to the initial single-span cavity has a pronounced effect on the Nusselt-number distribution on the floor of the cavity. The temperature and velocity distributions were also significantly influenced. The four-span cavity showed three convective cells instead of four as for the lowest Rayleigh number.

Effect of Combustor-Turbine Platform Misalignment on the Aerodynamics and Heat Transfer of an Axisymmetric Converging Vane Endwall at Transonic Conditions

This paper presents a detailed study on the effect of misalignment between the combustor exit and the nozzle guide vane endwall. The Nusselt number distribution and augmentation on an axisymmetric converging endwall as well as stage pressure losses were studied using experimental techniques and computational analysis. The analyzed endwall configurations are representative of the design intent and average off-design endwall configurations of a land-based high-pressure turbine nozzle guide vane. The studies were carried out at isentropic exit Mach number of 0.85, with an exit Reynolds number of 1.5 × 106 based on the true chord, and an inlet turbulence intensity of 16%. The experiment was conducted in a blowdown transonic linear cascade wind tunnel and an infrared camera was used to measure the surface temperature and subsequently the endwall heat transfer coefficient and Nusselt number distribution. Numerical computation analysis using ANSYS Fluent v.16 was used to provide further insight into the near-endwall flow field the predictions compared favorably to experimental data. The findings show that at the two configurations there exist uniquely different endwall secondary flow systems throughout the NGV stage. The interaction of separated flow at the combustor-turbine interface with the vane potential field results in additional secondary flow that is vastly different from that associated with classical endwall flows. This increased secondary flow in the misaligned configuration was marked by a 25% increase in NGV stage losses. The presence of separated flow and additional secondary flows also resulted in flow reattachment inside the vane passage which augmented heat transfer. The region upstream of the vane gage/throat showed heat transfer augmentation of up to 60%, while the endwall region downstream of the throat did not show any considerable heat transfer augmentation.

Effect of Freestream Motion on Heat Transfer Characteristics of Turbulent Offset Jet

The numerical simulation of turbulent offset jet flow has been carried out using k–ω shear stress transport (SST) model. The simulations have been done for the offset jet flow in the quiescent medium and also in the presence of an external stream. The effect of freestream velocity on the flow and heat transfer characteristics of turbulent offset jet has been reported. The offset ratio and Reynolds number of flow considered are 5.7 and 16,000, respectively. The presence of coflow stream has been found to reduce the entrainment of surrounding fluid into the jet which in turn reduces the heat transfer from the jet to the surrounding medium. The effect of freestream velocity on the important parameters like decay of the local maximum streamwise velocity, jet spread, reattachment length, velocity logarithmic profile, velocity defect law profile, decay of the local maximum streamwise temperature, variation of wall temperature, temperature similarity profile, and Nusselt number distribution has been discussed.

Numerical Investigation of a Three-Dimensional Laminar Mixed Convection Flows in Lid-Driven Cavity for Very Small Richardson Numbers

Laminar mixed convection in a three-dimensional lid driven cavity is numerically investigated. The top lid of the cavity is moving rightwards with a constant speed at a cold temperature. The bottom wall is maintained at an isothermal hot temperature, while the other vertical walls of the cavity are assumed to be insulated. In this study the mass diffusion was not taken into account and the fluid used was air. The flow and heat transfer behavior is studied for various Richardson number ranging from 5 × 10−5 to 3 × 10−4 at a fixed Prandtl number of 0.71 through analyzing the local Nusselt number distribution at different sections inside the cavity. Lewis number Le is assumed to be unity and the buoyancy ratio parameter N is equal to zero. Computations were done using an in-house code based on a finite volume method. The results showed a good agreement with previous two dimensional studies, while the three dimensional study gives different results at different sections inside the cavity. It is observed that, the average Nusselt number “Av Nu” on top and bottom surfaces decreases for all sections inside the cavity with increasing Richardson number. A correlation was formulated for each section on both walls for “Av Nu” as a function of “Ri” with a maximum error of 7.3%.

Computational and Experimental Study of Heat Transfer in Rotating Ribbed Radial Turbine Cooling Passages

This study investigates the effect of rotation on the Nusselt number distribution within ribbed radial turbine cooling passages representative of systems used in current jet engines. The results are unusual in that the cooling passage length to diameter ratio is engine representative and full distributions of local Nusselt number have been measured using the transient liquid crystal method. The results are compared to RANS CFD simulations and the level of agreement discussed in detail. A triple-pass serpentine passage is investigated, which includes 45° filleted rib-turbulators and 180° curved bends. The first two passes have an aspect ratio of 1:4 which are radially inward and outward respectively, with the final pass being radially outward with an aspect ratio of 1:2. The Reynolds, Rotation and Buoyancy numbers are all representative of a passage within a HP turbine blade of a gas turbine engine at 97000/108000, 0.081/0.088 and 0.052/0.035 respectively for the 1:4/1:2 aspect ratio passages. CFD simulations are found to give good predictions under stationary conditions however significant differences are observed when rotation is introduced. The Nusselt number distributions depend strongly on both rotation and upstream flow conditions created by the specific geometry.