Jet Impingement Heat Transfer Characteristics with Variable Extended Jet Holes under Strong Crossflow Conditions

In this paper, detailed flow patterns and heat transfer characteristics of a jet impingement system with extended jet holes are experimentally and numerically studied. The jet holes in the jet plate present an inline array of 16 × 5 rows in the streamwise (i.e., the crossflow direction) and spanwise directions, where the streamwise and spanwise distances between adjacent holes, which are normalized by the jet hole diameter (xn/d and yn/d), are 8 and 5, respectively. The jets impinge onto a smooth target plate with a normalized distance (zn/d) of 3.5 apart from the jet plate. The jet holes are extended by inserting stainless tubes throughout the jet holes and the extended lengths are varied in a range of 1.0d–2.5d, depending on the jet position in the streamwise direction. The experimental data is obtained by using the transient thermochromic liquid crystal (TLC) technique for wide operating jet Reynolds numbers of (1.0 × 104)–(3.0 × 104). The numerical simulations are well-validated using the experimental data and provide further insight into the flow physics within the jet impingement system. Comparisons with a traditional baseline jet impingement scheme show that the extended jet holes generate much higher local heat transfer levels and provide more uniform heat transfer distributions over the target plate, resulting in the highest improvement of approximately 36% in the Nusselt number. Although the extended jet hole configuration requires a higher pumping power to drive the flow through the impingement system, the gain of heat transfer prevails over the penalty of flow losses. At the same pumping power consumption, the extended jet hole design also has more than 10% higher heat transfer than the baseline scheme.

Role of Three-Dimensional Swirl in Forced Convection Heat Transfer Enhancement in Wavy-Plate-Fin Channels

The influence of swirl flow on enhanced forced convection in wavy-plate-fin cores has been investigated. Three-dimensional computational simulations were carried out for steady-state, periodically developed flow of air (Pr ~ 0.71) with channel walls subject to constant-uniform temperature and flow rates in the range 50 = Re = 4000. The recirculation that develops in the wall troughs and grows to an axial helix is scaled by the Swirl number Sw. As Sw increases, tornado-shaped vortices appear in the wave trough region mid-channel height, then extend longitudinally to encompass majority of the flow channel. As shown by the local wall-shear and heat transfer coefficient variations, the boundary-layer thinning upstream of the wave peak assists to intensify the momentum and heat transfer. However, the flow recirculation in wave trough impedes the local heat transfer at low Sw due to flow stagnation but promotes it at high Sw because of swirl-augmented fluid mixing. Swirling flows also create pressure drag that contributes substantively to the overall pressure loss. Its proportion grows as Sw, corrugation severity, and fin spacing increases to as much as 80% of the total pressure drop. The fin-wall curvature-induced secondary circulation nevertheless produces significantly enhanced convection, and more so in flows with higher Sw. It is quantified by Ff (or j), which is seen to increase log-linearly as fin corrugation aspect ratio and/or fin spacing ratio increases; the influence of cross-section aspect ratio is found to be marginal.

Testing of Heat Transfer Coefficients and Frictional Losses in Internally Ribbed Tubes and Verification of Results through CFD Modelling

This paper presents experimental determination of the heat transfer coefficient and the friction factor in an internally rifled tube. The experiment was carried out on a laboratory stand constructed in the Department of Energy of the Cracow University of Technology. The tested tube is used in a Polish power plant in a supercritical circulating fluidized bed (CFB) boiler with the power capacity of 460 MW. Local heat transfer coefficients were determined for Reynolds numbers included in the range from ~6000 to ~50,000, and for three levels of the heating element power. Using the obtained experimental data, a relation was developed that makes it possible to determine the dimensionless Chilton–Colburn factor. The friction factor was also determined as a function of the Reynolds number ranging from 20,000 to 90,000, and a new correlation was developed that represents the friction factor in internally ribbed tubes. The local heat transfer coefficient and the friction factor obtained during the testing were compared with the CFD modelling results. The modelling was performed using the Ansys Workbench application. The k-ω, the k-ε and the transition SST (Share Stress Transport) turbulence models were applied.

AERODYNAMICS AND HEAT EXCHANGE OF SINGLE END TUBE DURING EXTERNAL FLOW

In Ukraine, the safety of modern thermal power plants depends on the reliable operation of the equipment installed on them. Unfortunately, the technical condition of the chimneys is not properly maintained. Of course, the modernization of basic equipment (boilers, switching to another type of fuel) leads to a decrease in the temperature of the exhaust gases. An important aspect to maintain the condition of the chimneys is to maintain the moisture of the exhaust gases. An important feature of the external flow of chimneys are large Reynolds numbers Re = wd/n, which reach 106 and more. In the thermal calculation only the average heat transfer coefficient on the outer surface of the pipe is usually determined, and the features of aerodynamics and local heat transfer due to the conicity of the pipe are not taken into account. The work is devoted to the study of aerodynamics and heat transfer in the air flow of a single conical chimney. The method of computer modeling with numerical integration of equations of motion and energy was used in the research. At the first stage, the single pipe with the uniform flow profile is considered. Further, the influence of the surrounding infrastructure on the aerodynamics and heat transfer of a single conical tube is studied. The single conical vertical pipe with 40 m height, 1.7 m diameter at the base and 0.85 m in the mouth was used for the calculation. The computer model was calculated in the ANSYS2020-R1 program. The model is developed in a homogeneous area with the air environment. In order to obtain reliable results, the study was conducted to obtain the optimal set of the grid parameters for the heat transfer conditions. The grids with parameters that affect the distance of the first node from the cylinder wall (options a, b, c, d) and the rate of increase in the size of the elements as they move away from the area of interest (Growth rate GR) were studied. The type of the cylindrical pipe with constant diameter of 1.7 m has been chosen to analyze the sensitivity and to check the grid. The turbulence model has been choosen as the following: RNG k-ε model which is common for the tasks of this class, the Enhanced Wall Function, the solution algorithm for the connection of the velocity pressure in stable flows Simplex. It is determined that in case if the distance between the first node from the cylinder wall and the area of interest (Growth rate GR) is more than 8 mm, instability and deviation of the obtained data from the values of the average coefficient of more than 20% appears. As a result of the research, the parameter grid area matching to the “2d” option of table 1 has been selected, i.e.: GR = 1.1, h = 8 mm. In the study of aerodynamics and heat transfer, the conical tube is conventionally divided into 22 sections (with 1 m height each). The case of uniform flow velocity in front of the pipe has been considered. As seen, the maximum value of the heat transfer coefficient is in the Zone(21-22). The research shows that oncoming flow velocity of 25 m/s causes the average value of heat transfer coefficient of the conical pipe 62.5 W/m2K, and 61.1 W/m2K according to the known formula . This indicates a small effect of taper on the average heat transfer of the entire pipe. In the calculations, three types of surrounding areas are considered: A - open coasts of seas, lakes and reservoirs, rural areas, including buildings less than 10 m high; B - urban areas, forests and other areas, evenly covered with obstacles higher than 10 m; C - urban areas with dense buildings with buildings higher than 25 m. Thus, the wind speed profiles for different types of terrain are nonlinear. The wind speed profile in front of the pipe (type of terrain) has a significant effect on the heat transfer coefficient. This confirms the need to take into account the type of terrain and the velocity profile in front of the pipe for local heat transfer.

Experimental study of the heat transfer of two parallel impinging jets

Local and integral characteristics of heat transfer are obtained at varying the Reynolds number Re = 5500, 11000, the distance between the jets y/D = 1.8, and the distance from the jets to the surface z/D = 0.5-10 for the system of two identical impinging jets. It is found in experiments that the effect of an adjacent jet leads to enhancement of local heat transfer at large distances between the nozzles and the barrier. It is also shown that an increase in the Re number increases integral heat transfer, and, at the same time, weakens the inter-jet interaction. The paper analyzes the scenarios of the behavior of local and integral heat transfer depending on the geometric and flow parameters of the system of two circular turbulent jets.

Non-stationary convective heat transfer in an air synthetic impinging jet. Experiment and numerical simulation

An unsteady local heat transfer in an air synthetic non-steady-state jet impingement onto a flat plate with a variation of the Reynolds number, nozzle-to-plate distance and pulses frequency is experimentally and numerically studied. Measurements of the averaged and pulsating heat transfer at the stagnation point are conducted using a heat flux sensor. The axisymmetric URANS method and the Reynolds stress model are used for numerical simulations. For local values of heat transfer, zones with the maximum instantaneous value of heat flux and heat transfer coefficient are identified. The heat transfer increases at relatively low nozzle-to-plate distances (H/d ≤ 4). The heat transfer decreases at high distance from the orifice and target surface. An increase in the Reynolds number causes reduction of heat transfer.