Submerged jet impinging asymmetrically onto a rotating surface

Experimental investigation on convective heat transfer of inclined jets impinging on the rotating cylindrical surface in the confined space

Heat transfer over rotating surfaces is of particular interest in rotating machinery such as gas turbine engines. The rotation of the gas turbine disc creates a radially outward flow on the disc surface, which may lead to ingress of hot gases into the narrow cavity between the disc and the stator. Impingement of cooling jet is an effective way of cooling the disc and countering the ingress of the hot gases. Present study focusses on investigating the effect of introducing pin-fins over the rotating disc on the heat transfer. The jet Reynolds number has been varied from 5000 to 18000, and the rotating Reynolds number has been varied from 5487 to 12803 for an aluminum disc of thickness 6.35mm and diameter 10.16 cm, over which square pins have been arranged in an inline fashion. Steady state temperature measurements have been taken using thermocouples embedded in the disc close to the target surface, and area average Nusselt number has been calculated. The effects of varying the height of the pin-fins, distance between nozzle and the disc surface and the inclination of the impinging jet with the axis of rotation have also been studied. The results have been compared with those for a smooth aluminum disc of equal dimensions and without any pin-fins. The average Nusselt number is significantly enhanced by the presence of pin fins. In the impingement dominant regime, where the effect of disc rotation is minimal for a smooth disc, the heat transfer increases with rotational speed in case of pin fins. The effect of inclination angle of the impinging jet is insignificant in the range explored in this paper (0° to 20°).

Many engineering applications consist of rotating components that experience high-heat load. Applications like the gas turbine engine consist of rotating disks and the study of heat transfer over such rotating surfaces is of particular interest. In the case of gas turbines, the disk also needs to be protected from the ingress of hot turbine gases caused by the low-pressure region created due to the radially outward pumping of fluid close to the rotating surface. The present experimental study investigates the effects of introducing pin-fins on heat transfer over surface of a rotating gas-turbine disk. The experiments were conducted at rotational Reynolds numbers (ReR) of 5487–12,803 based on the disk diameter (D) and jet Reynolds numbers (Re) of 5000–18,000 based on the jet diameter. The effects of nozzle to target spacing (z/d=2−6), eccentricity of impingement (ε = 0−0.67), angle of impingement—both toward and away from the center (θi and θo=0 deg−20 deg), and the pin fin height (Hf=3.05 mm−19.05 mm) were studied. Steady-state temperature measurements were taken using thermocouples embedded in the disk close to the target surface, and area average Nusselt number (Nu) was calculated. The results have been compared with those for a smooth aluminum disk of equal dimensions and without any pin-fins. The average Nu was significantly enhanced by the presence of pin-fins. The enhancement was higher for lower values of Re, and the maximum enhancement was found to be 3.9 times that of a smooth disk for Re=5000. In the impingement dominant regime, the effect of disk rotation was minimal for a smooth disk, but the heat transfer increased with rotational speed in case of pin-fins. There was no impact of eccentricity on Nu for ε = 0 and 0.33. For ε = 0.67, the maximum reduction in enhancement over a smooth surface (21.95%) was observed when compared with coaxial impingement for stationary impingement for Re=18,000 and z/d=4. The effect of inclination angle was insignificant, and no clear trend could be established. Higher heat transfer rates were observed for z/d=6 with the increasing Re, and this effect diminished with the increase in the rotational speed. With the increase in pin-fin height, especially at higher values of Re, there was in increase in the value of Nu. Qualitative visualization of flow field has been performed for smooth and the pin-fin case using the commercial simulation package Ansys Fluent to further understand the flow features that result in the heat transfer enhancement.

Fluid flow, heat transfer and entropy generation characteristics of micro-pipes are investigated computationally by considering the simultaneous effects of pipe diameter, wall heat flux and Reynolds number in detail. Variable fluid property continuity, Navier-Stokes and energy equations are numerically handled for wide ranges of pipe diameter (d = 0.50–1.00 mm), wall heat flux (q''= 1000–2000 W/m2) and Reynolds number (Re = 1 – 2000), where the relative roughness is kept constant at e/d = 0.001 in the complete set of the scenarios considered. Computations indicated slight shifts in velocity profiles from the laminar character at Re = 500 with the corresponding shape factor (H) and intermittency values (γ) of H = 3.293→3.275 and γ = 0.041→0.051 (d = 1.00→0.50 mm). Moreover, the onset of transition was determined to move down to Retra = 1,656, 1,607, 1,491, 1,341 and 1,272 at d = 1.00, 0.90, 0.75, 0.60 and 0.50 mm, respectively. The impacts of pipe diameter on friction mechanism and heat transfer rates are evaluated to become more significant at high Reynolds numbers, resulting in the rise of energy loss data at the identical conditions as well. In cases with low pipe diameter and high Reynolds number, wall heat flux is determined to promote the magnitude of local thermal entropy generation rates. Local Bejan numbers are inspected to rise with wall heat flux at high Reynolds numbers, indicating that the elevating role of wall heat flux on local thermal entropy generation is dominant to the suppressing function of Reynolds number on local thermal entropy generation. Cross-sectional total entropy generation is computed to be most influenced by pipe diameter at high wall heat flux and low Reynolds numbers.

Measurements of the local heat transfer distribution on smooth and roughened surfaces under an array of angled impinging jets are presented. The test rig is designed to simulate impingement with cross-flow in one direction which is a common method for cooling gas turbine components such as the combustion liner. Jet angle is varied between 30, 60, and 90 degrees as measured from the impingement surface, which is either smooth or randomly roughened. Liquid crystal video thermography is used to capture surface temperature data at five different jet Reynolds numbers ranging between 15,000 and 35,000. The effect of jet angle, Reynolds number, gap, and surface roughness on heat transfer efficiency and pressure loss is determined along with the various interactions among these parameters. Peak heat transfer coefficients for the range of Reynolds number from 15,000 to 35,000 are highest for orthogonal jets impinging on roughened surface; peak Nu values for this configuration ranged from 88 to 165 depending on Reynolds number. The ratio of peak to average Nu is lowest for 30-degree jets impinging on roughened surfaces. It is often desirable to minimize this ratio in order to decrease thermal gradients, which could lead to thermal fatigue. High thermal stress can significantly reduce the useful life of engineering components and machinery. Peak heat transfer coefficients decay in the cross-flow direction by close to 24% over a dimensionless length of 20. The decrease of spanwise average Nu in the crossflow direction is lowest for the case of 30-degree jets impinging on a roughened surface where the decrease was less than 3%. The decrease is greatest for 30-degree jet impingement on a smooth surface where the stagnation point Nu decreased by more than 23% for some Reynolds numbers.

In a crystal growth system, the crystal quality is greatly affected by the coupling properties between unsteady melt flow and thermal transfer. In this paper, an improved lattice Bolzmann method is proposed. This incompressible axisymmetric model based method transforms the fluid equations of cylindrical coordinate into those of the two-dimensional Cartesian coordinate and constructs the evolutionary relationship of the external force terms, such as rotational inertia force and the thermal buoyancy. In the unsteady melt, the temperature distribution and the rotational angular velocity are determined based on the D2Q4 model and the velocity of axisymmetric swirling fluid is calculated based on the D2Q9 model. The mirror bounce format is adopted as the boundary conditions of the free surface and the axis symmetry. For the remaining boundary conditions, the non-equilibrium extrapolation format is used. In the simulation, 12 sets of flow function results are obtained by choosing different sets of Grashof number and Reynolds number. By comparing with the finite crystal growth results, the effectiveness of the proposed method can be shown. Furthermore, by studying the convection shape and the temperature distribution of the melt under coupling between high Grashof number and high Reynolds number, it can be concluded that the thermal coupling properties and flow in the unsteady melt relate to Grashof number and Reynolds number. By adjusting the high Reynolds number generated by the crystal and crucible rotation, the strength of the forced convection in the melt can be changed. Therefore, the natural convection in the melt can be suppressed effectively and the temperature distribution results can be improved significantly. In addition, it is worth mentioning that the findings in this paper can be straightforwardly extended to the silicon single crystal growth experiment by turning the dimensionless crystal rotation Reynolds number and crucible rotation Reynolds number into the actual rotation speed.

Limited available data suggest a substantial impact of Mach number on the heat transfer from an array of jets impinging on a surface at fixed Reynolds number. Many jet array heat transfer correlations currently in use are based on tests in which the jet Reynolds number was varied by varying the jet Mach number. Hence, this data may be inaccurate for high Mach numbers. Results from the present study are new and innovative because they separate the effects of jet Reynolds number and jet Mach number for the purposes of validating and improving correlations that are currently in use. The present study provides new data on the separate effects of Reynolds number and Mach number for an array of impinging jets in the form of discharge coefficients, local and spatially averaged Nusselt numbers, and local and spatially averaged recovery factors. The data are unique because data are given for impingement jet Mach numbers as high as 0.60 and impingement jet Reynolds numbers as high as 60,000, and because the effects of Reynolds number and Mach number are separated by providing data at constant Reynolds number because the Mach number is varied, and data at constant Mach number because the Reynolds number is varied. As such, the present data are given for experimental conditions not previously examined, which are outside the range of applicability of current correlations.

In the present paper, the flames from DJHC burner, imitating MILD (Moderate and Intense Low Oxygen Dilution) combustion, are simulated using PDF transport modeling. Two different solution approaches have been used to resolve the joint composition PDF. First, a Lagrangian approach is used to solve the joint composition PDF, while in the second approach, the approximate solution is achieved by using presumed shape PDF and DQMOM-IEM modeling known as Multi-Environment Eulerian PDF (MEPDF). A quantitative comparison of the predictions from these two solution methods has been performed for two different jet Reynolds number, i.e. Re = 4100 & 8800. Moreover, the effect of molecular diffusion is also explored by comparing the predictions using different micro-mixing models such as Coalescence Dispersion (CD), Euclidean Minimum Spanning Tree (EMST), and Interaction-by-Exchange-with-Mean (IEM) model. The obtained numerical predictions from both approaches are compared with the experimental data to highlight the accuracy as well as the predictive capability of these models. In the case of low Reynolds number (Re = 4100), it is observed that the mean axial velocity and turbulent kinetic energy profiles are in good agreement with the measurements while the temperature profiles are slightly over-predicted in the downstream region. Although MEPDF results are in good agreement with the LPDF results, both the model predictions tend to exhibit discrepancies at higher Reynolds number.

An experimental study of the mean wake flow field and its stability has been carried out in the far wake of circular cylinders at a Mach number of 6. The Reynolds numbers ranged from 200 to 4000 with a few measurements at higher Reynolds number. Pitot pressure, static pressure and mean flow hot wire measurements were done at many axial stations behind cylinders up to (x/d) = 2400. The inner wake formed from the cylinder boundary layers is laminar and loses its identity within the first 60 diameters or less depending on the Reynolds numbers so that only the outer wake, caused by the bow shock, has to be considered. Within a certain region the experimental results compare well with linear laminar theory, but the wake profiles are not similar up to the farthest downstream station (x/d = 2400). At four Reynolds numbers strong deviations from steady laminar behavior were observed far behind the cylinder, indicating breakdown of the flow because of non-linear instability effects. In the instability study hot wire fluctuation measurements were made over the whole frequency range (f = 1 - 320 KC) and also at particular frequencies in a band width of 1 KC up to x/d = 12000 at the lowest Reynolds number. Two instability regions were found and investigated: the linear growth region and the non-linear region. In the linear region there is quite a close correspondence with linear stability theory. The onset of non-linearity is characterized by the simultaneous strong deviation of the mean flow from laminar steady behavior, the increase of the fundamental frequency fluctuation component on the wake axis and the sudden rise of the first harmonic frequency component. The non-linear region is compared with the non-linear wake region at low speeds behind a flat plate. On the basis of these measurements a tentative picture is given of the onset of non-linearity and/or transition in the inner and outer wake behind blunt bodies at hypersonic speeds.

Low Reynolds number flow through nozzle-diffuser elements in valveless micropumps

The amount of cooling air assigned to seal high pressure turbine rim cavities is critical for performance as well as component life. Insufficient air leads to excessive hot annulus gas ingestion and its penetration deep into the cavity compromising disk or cover plate life. Excessive purge air, on the other hand, adversely affects performance. This paper is Part-2 of the authors’ work on ingestion reported last year [1]. Whereas, the main focus of that investigation was to qualitatively describe ingestion driven by annulus circumferential pressure asymmetry under constant annulus conditions and rotational speed, in this paper, the research team investigated the variation of annulus circumferential pressure fluctuation and rotational speed on the double overlap platform rim seal cavity reported in Part-1, and mapped out the resulting non-dimensional minimum sealing flow (minimum value of Cw or Cw,min) as it relates to entrained ingestion in the absence of cavity cooling flow (Cw,ent). As was done in Part-1, the runs were made with 3D CFD in setup/run mode option using Fine/Turbo. At two rotational speeds, annulus conditions were varied by reducing turbine inlet pressure (i.e. mass flow) from the baseline operating condition, and the resulting pressure fluctuation was quantified. In addition, a preliminary investigation to assess the aft-located mixing plane steady state solution for this study was performed. The results yielded the linear decrease in Cw,ent at fixed rotational Reynolds number as annulus Reynolds number was decreased. Moreover, the rate of change in entrained flow sharply increases with increase in rotational Reynolds number. As annulus mass flow is reduced to a critical value defined by annulus-to-rotational Reynolds number ratio, the CFD prediction for Cw,ent converges to the turbulent boundary layer entrainment solution for the rotor, and Cw,min reverts to the rotational Reynolds number dominating region. The results from this study were compared to what has been observed by a previous study for a single overlap platform geometry. The resulting design curve allows insight in relating cavity purge flow requirements versus turbine cycle parameters which could lead to better efficiency.

Convective mass transfer from a horizontal rotating large-diameter cylinder

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.

Local and global effects of cylindrical vortex generators on the mass-transfer distributions over the four active walls of a square, rib-roughened rotating duct with a sharp 180-deg bend are investigated. Cylindrical vortex generators (rods) are placed above, and parallel to, every other rib on the leading and trailing walls of the duct so that their wake can interact with the shear layer and recirculation region formed behind the ribs, as well as the rotation-generated secondary e ows. Local increases in near-wall turbulence intensity resulting from these interactions give rise to local enhancement of mass (heat)transfer. Measurements are presented for duct Reynolds numbersReintherange5 £ 10 3‐3 £ 10 4 andforrotationnumbersin therange0 ‐0.3. Therib height-to-hydraulic diameter ratio e/Dh is e xed at 0.1, and the rib pitch-to-rib height ratio P/e is 10.5. The vortex generator rods have a diameter-to-rib height ratio d/e of 0.78, and the distance separating them from the ribs relative to the rib height s/e is 0.55. Mass-transfer measurements of naphthalene sublimation have been carried out using an automated acquisition system and are correlated with heat transfer using the heat/mass transfer analogy. The results indicate that the vortex generators tend to enhance overall mass transfer in the duct, compared to the case where only ribs are present, both before and after the bend at high Reynolds and rotation numbers. Local enhancements of up to 30%are observed on all fourwalls oftheduct. At low Reynolds numbers, forexample, 5 ££ 10 3 , the insertion of the rods often leads to mass-transfer degradation. At high Reynolds numbers, for example, 3 ££ 10 4 , the enhancement due to the rods occurs on the surfaces stabilized by rotation (trailing edge on the inlet pass and leading edge on the outlet pass )and the side walls. The enhancement is more pronounced as the rotation number is increased. The detailed measurements in a ribbed duct with vortex-generator rods clearly show localized regions of enhanced mass (heat) transfer at Reynolds and rotation numbers within the envelope of practical interest for gas-turbine blade cooling applications.

Forced convection on a rotating cylinder with an incident air jet