Experimental and Computational Study on Effect of Vanes on Heat Transfer and Flow Structure of Swirling Impinging Jet

The study focuses on heat transfer performance and flow structure associated with swirling jet on a flat target surface. The analysis is carried out with helicoid inserts of swirl number S = 1.3 by varying the number of vanes with Reynolds number between 11200 and 35600. The comparison of swirling jet with circular jet is carried out on its heat transfer performance. The heat transfer and flow structure are visualized using thermo-chromic liquid crystal sheet and oil film technique respectively. The numerical simulation is also performed at Re = 24700 for H/D distance between 1 and 4 using computational fluid dynamics. The heat transfer results reveal that the presence of axial recirculation zone at Re = 29800 and 35600 for the triple helicoid affects the uniformity of heat transfer distribution at 0 < X/D < 1.5 at H/D = 3. The axial component of velocity with respect to swirling jet is less than zero in the stagnation area and it increases at 0.57 < r/D < 0.97 for single vane and 0.63 < r/D < 0.97 for double and triple vanes. While the steep increase in tangential velocity of the triple vane jet is apparent at 0 < r/D < 0.5 at H/D = 2 and 3, the maximum value of point radially shifts inward towards the jet. The location of maximum turbulent kinetic energy approaching the surface at about r/D = 0.9 - 1.2 which characterizes the swirling jet at H/D = 2

Impingement of coaxial jet on convex element for confined and unconfined flow

Jet impingement is most effective and active method for cooling and heating of any surface or system. The ability of jet impingement is greatly influenced by nozzle configuration and other dimensional and non-dimensional parameters. Impinging coaxial swirl jet generates interesting flow filed on any test surface and influences both pressure and heat distribution on impinging surfaces. In present study, an experimental investigation is carried to analyze the effects of turbulent coaxial swirl jet on the pressure distribution (PC & PCO) on convex element. For better and acceptable results, the desirable parameters are identified from previous research works. The present experimental result highlights the independency of pressure coefficient (PC) for jet-Reynolds number (Re=70000 to 45000), effect of circumferential angle (θ) or inclination of test element, effect of jet exit to test element distance (Z/dh) and effect of confinement on PC & PCO pattern on a convex test element. The higher pressure coefficient value are obtained at lower Z/dh = 1 & at θ = 15° to 12°and significant drop in the values are seen with increase in the Z/dh & θ. At θ = 20° to 30° the value of PC & PCO reaches to negative magnitude. The use of confinement tube enhancementthe pressure distribution (PC & PCO) by 61% to 64% is seen for the same flow conditions

Two Phase Flow, Phase Change and Numerical Modeling

The heat transfer and analysis on laser beam, evaporator coils, shell-and-tube condenser, two phase flow, nanofluids, complex fluids, and on phase change are significant issues in a design of wide range of industrial processes and devices. This book includes 25 advanced and revised contributions, and it covers mainly (1) numerical modeling of heat transfer, (2) two phase flow, (3) nanofluids, and (4) phase change. The first section introduces numerical modeling of heat transfer on particles in binary gas-solid fluidization bed, solidification phenomena, thermal approaches to laser damage, and temperature and velocity distribution. The second section covers density wave instability phenomena, gas and spray-water quenching, spray cooling, wettability effect, liquid film thickness, and thermosyphon loop. The third section includes nanofluids for heat transfer, nanofluids in minichannels, potential and engineering strategies on nanofluids, and heat transfer at nanoscale. The forth section presents time-dependent melting and deformation processes of phase change material (PCM), thermal energy storage tanks using PCM, phase change in deep CO2 injector, and thermal storage device of solar hot water system. The advanced idea and information described here will be fruitful for the readers to find a sustainable solution in an industrialized society

Computational Investigation of Swirling Jet Impingement in a Concentrated Solar Tower Receiver

With growing concern of climate change and environmental pollution the need for better renewable technologies is a necessity. Solar energy shows the most promise in meeting global energy needs and competing with fossil fuels economically. Currently solar power is generated with photovoltaic (PV) panels and stored in batteries. The disadvantages of PV are expensive batteries, limitations on panel efficiencies and electrical grid considerations to balance electricity generation. Concentrated solar power (CSP) is an alternative that addresses PV limitations and shows potential in a hybrid power generation mix, especially because of its thermal storage capabilities and ability to provide process heat directly. CSP consists of a variety of systems. Of all available CSP technologies, solar power towers (SPT) show potential to reach high temperatures and effectively store thermal energy. For SPT the central receiver shows promise for improvement in effectively capturing heat. Of the many methods available to improve heat transfer, jet impingement with swirl can improve heat transfer for the receiver fluid. Jet impingement heat transfer is well known to enhance local heat transfer because of the local increase in the heat transfer coefficient and Nusselt number. Swirling flows have also shown to enhance heat transfer for internal pipe flow arrangements and other heat transfer applications. The effect of swirl and jet impingement are not often considered cumulatively as in the current study. For a proposed solar receiver design, a swirling impinging jet is proposed to enhance heat transfer. The flow behaviour is investigated numerically using computational fluid dynamics (CFD). Ansys Fluent is used to model the flow behaviour and to validate the model with available experimental results. From the validation study the Transition Shear-Stress-Transport turbulence model is shown to predict jet impingement the best. A 2D axisymmetric assumption is however shown to not predict the heat transfer well while a costly full 3D transient Large Eddy simulation does. As LES is too expensive for use in a parametric investigation, both 2D and 3D RANS simulations were used as an engineering tool to improve and optimise heat transfer, keeping in mind their shortcomings. Swirling jet impingement is further investigated for a curved impingement surface. This is the first investigation of its kind where swirl, jet impingement and a curved impingement surface are considered. From the validation study, a CFD model is used to investigate how curvature affects heat transfer. The parameters show that surface curvature has a large effect on heat transfer and it is shown that a potential optimal curvature exists for the unique flow arrangement. A surrogate optimisation model is used from the numerical results to improve the design. To provide a realistic heat source on the solar receiver, Monte Carlo ray tracing (MCRT) is used to model the heliostat field. The MCRT model can better predict the solar flux distribution on the receiver absorbing surface. The solar flux distribution is an important consideration for the receiver design. The CFD model of the receiver showed that while swirling jet impingement did not increase the outlet temperature of the heat transfer fluid, it did however show potential to reduce the receiver’s maximum surface temperature and as well as radiation losses. The thermal enhancements made do however come at the cost of an increased pressure drop.Dissertation (MEng)--University of Pretoria, 2021.Mechanical and Aeronautical EngineeringMEngUnrestricte