Impingement heat transfer at a circular cylinder due to an offset or non-offset slot jet

Heat transfer coefficients were measured on a circular cylinder subjected to the crossflow impingement of a slot jet. In one set of experiments, the symmetry plane of the jet was aligned with the axis of the cylinder, while in other experiments the jet was offset from the cylinder. In addition to the offset, parametric variations were also made for the width of the jet-inducing slot, the distance between the slot and the cylinder, and the Reynolds number. Supplementary flow visualization experiments showed that even in the presence of offset, the jet impinged on the cylinder, although not at the cylinder apex as in the aligned case. It was found that the heat transfer coefficient increased with slot width and Reynolds number but decreased with slot-to-cylinder separation distance and offset. The effect of offset is accentuated for narrow slots and at small slot-to-cylinder separation distances. The largest measured offset-related reduction in the heat transfer coefficient was slightly in excess of 50%.     

The Impingement Heat Transfer Data of Inclined Jet in Cooling Applications: A Review

On the impingement heat transfer data,the experimental studies of air and liquid jets impingement to the flat surfaces were collected and critically reviewed.The oblique impingements of both single circular and planar slot jets were considered in particular.The review focused on the surface where the jet impingement cooling technique was utilized.The nozzle exit Reynolds numbers based on the hydraulic diameter varied in the range of 1,500–52,000.The oblique angles relative to the plane surface and the dimensionless jet-to-plate spacing vary in the range of 15°–90°and 2–12 respectively.The review suggested that the magnitude of maximum heat transfer shifted more for air jets compared with the liquid jets.The drop in the inclination angle and the jet-to-plate separation led to the increase in the asymmetry of heat transfer distribution.The displacement of maximum Nusselt number(heat transfer)locations was found to be sensitive to the inclination angle and the smaller jet-to-plate distance.Also,the Nusselt number correlations proposed by various researchers were discussed and compared with the results of the cited references.     

MIXED-CONVECTIVE COOLING OF AN ISOTHERMAL HOT SURFACE BY CONFINED SLOT JET IMPINGEMENT

The flow and heat transfer characteristics in the cooling of a heated surface by impinging slot jets have been investigated numerically. Computations are done for vertically downward-directed two-dimensional slot jets impinging on a hot isothermal surface at the bottom and confined by a parallel adiabatic surface on top. Some computations are also performed where the jet is vertically upward, with an impingement plate at the top. The principal objective of this study is to investigate the associated heat transfer process in the mixed-convective regime. The computed flow patterns and isotherms for various domain aspect ratios (4–10) and for a range of jet exit Reynolds numbers (100–500) and Richardson numbers (0–10) are analyzed to understand the mixed-convection heat transfer phenomena. The local and average Nusselt numbers and skin friction coefficients at the hot surface for various conditions are presented. It is observed that for a given domain aspect ratio and Richardson number, the average Nusselt number at the hot surface increases with increasing jet exit Reynolds number. On the other hand, for a given aspect ratio and Reynolds number, the average Nusselt number does not change significantly with Richardson number, indicating that the buoyancy effects are not very significant in the overall heat transfer process for the range of jet Reynolds number considered in this study. Also, for the same problem configuration, the average Nusselt number does not change significantly when the jet is moving upward or downward.     

Mixed convection heat transfer in a differentially heated square enclosure with a conductive rotating circular cylinder at different vertical locations

In this investigation, a numerical simulation using a finite volume scheme is carried out for a laminar steady mixed convection problem in a two-dimensional square enclosure of width and height (L), with a rotating circular cylinder of radius (R = 0.2 L) enclosed inside it. The solution is performed to analyze mixed convection in this enclosure where the left side wall is subjected to an isothermal temperature higher than the opposite right side wall. The upper and lower enclosure walls are considered adiabatic. The enclosure under study is filled with air with Prandtl number is taken as 0.71. Fluid flow and thermal fields and the average Nusselt number are presented for the Richardson numbers ranging as 0, 1, 5 and 10, while Reynolds number ranging as 50, 100, 200 and 300. The effects of various locations and solid-fluid thermal conductivity ratios on the heat transport process are studied in the present work. The results of the present investigation explain that increase in the Richardson and Reynolds numbers has a significant role on the flow and temperature fields and the rotating cylinder locations have an important effect in enhancing convection heat transfer in the square enclosure. The results explain also, that the average Nusselt number value increases as the Reynolds and Richardson numbers increase and the convection phenomenon is strongly affected by these parameters. The results showed a good agreement with further published works.     

Vortex shedding and heat transfer dependence on effective Reynolds number for mixed convection around a cylinder in cross flow

The influence of surface heating of a circular cylinder on the wake structure and heat transfer in the range of Reynolds number (Re) for which parallel vortex shedding occurs, is investigated numerically for different values of the buoyancy parameter, Gr. The role of buoyancy induced baroclinic vorticity on the wake formation is addressed in the present study. The variation of Strouhal number and Nusselt number with the ’effective Reynolds number’, is analyzed for different values of cylinder to free stream temperature ratio. Both Strouhal number and the rate of heat transfer increases monotonically with the increase of the effective Reynolds number. The validity of the correlations, which have been established by several authors, between the effective Reynolds number and Strouhal/ Nusselt number for forced convection, is examined in the mixed convection regime. The curves between the effective Reynolds number and the computed data for Strouhal number and Nusselt number do not collapse for the range of temperature ratio considered here. The flow field is found to be asymmetric and the cylinder experiences a negative lift. The drag coefficient increases steadily with the rise of surface temperature.     

Correlation for laminar mixed convection from a rotating cylinder

The problem of laminar mixed convection from a rotating isothermal cylinder was solved numerically. A correlation for the average Nusselt number as a function of Reynolds number and buoyancy parameter is proposed. The correlation gives an accurate estimate of the Nusselt number over the range of Reynolds numbers and buoyancy parameter values studied.     

Study of heat transfer for a pair of rectangular jets impinging on an inclined surface

Critical design parameters in jet impingement heat transfer like nozzle hydraulic diameter, jet angle and velocity, physical properties of the fluid, and nozzle-to-target plane spacing are the subject. This paper identifies the dominant fluid-thermal characteristics of a pair of rectangular air jets impinging on an inclined surface. Heat transfer modes and flow characteristics are studied with eight different Reynolds numbers ranging from 500 to 20 000. Local and average Nusselt numbers are evaluated with two different boundary conditions on three specified lines located on the inclined surface. The correlation between stagnation Nusselt number and Reynolds number is presented. Turbulent intensity and wall y⁺ distributions are compared on three lines parallel to the incline. The effect of jet impingement angle on local and average Nusselt number is also documented. Finally, a correlation between the average Nusselt number, nozzle exit Reynolds number and the jet angle is documented.     

Laminar forced convection heat transfer to ordered and disordered single rows of cylinders

We study laminar forced convection heat transfer to or from a single row of equidistantly and non-equidistantly spaced parallel cylindrical wires, perpendicular to the flow direction. We report average Nusselt numbers as a function of geometry and flow conditions, for open frontal area fractions between 0.04 and 0.95, Prandtl numbers between 0.7 and 10, and Reynolds numbers (based on the wire diameter and the free stream velocity) between 0.001 and 600. For equidistantly spaced rows of cylindrical wires we propose a general analytical expression for the average Nusselt number as a function of the Reynolds number, Prandtl number and the open frontal area fraction, as well as asymptotic scaling rules for small and large Reynolds. For all studied Prandtl numbers, equidistant rows exhibit decreasing average Nusselt numbers for decreasing open frontal area fractions at low Reynolds numbers. For high Reynolds numbers, the Nusselt number approaches that of a single cylinder in cross-flow, independent of the open frontal area fraction. For equal open frontal area fractions, the Nusselt number in non-equidistant rows is lower than in equidistant rows for intermediate Reynolds numbers. For very low and high Reynolds numbers, non-uniformity does not influence heat transfer.     

Numerical Analysis of Heat Transfer from a Moving Surface Due to Impingement of Slot Jets

Heat transfer from a moving surface with uniform wall temperature due to impingement of series of slot jets has been investigated numerically. In the present paper, transition–shear stress transport model has been used for numerical simulations, which can predict the heat transfer in laminar as well as turbulent flows. This model is adopted here to study the transport phenomenon and predict the transition from laminar to turbulent flow seamlessly under different surface velocities. The present model with stationary surface is validated with the correlation given by Martin for series of slot jets. It has also shown good agreement with existing data for both laminar and turbulent slot jets, and is further studied to understand the heat transfer under wide range of flow conditions and the effect of surface velocity on flow regime. The range of Reynolds number is from 100 to 5,000, whereas surface velocity varied up to six times the jet velocity at the nozzle exit. It has been observed that at high surface velocities the heat transfer from the moving wall is more than stationary case. The transition from laminar to turbulent regime is found to be starting at a Reynolds number of 400 and turns completely turbulent at a Reynolds number of 3,000. Q-criterion is used to confirm the transition zone by observing the breaking of vortices at higher Reynolds number.     

Heat transfer due to double laminar slot jets impingement onto an isothermal wall within one side closed long duct

In this study, heat transfer due to double impinging vertical slot jets onto an isothermal wall was investigated numerically for laminar flow regime. Navier–Stokes and energy equations were discretized with a finite volume procedure on a non-staggered grid arrangement using SIMPLEM (SIMPLE-Modified) algorithm. The effect of the jet Reynolds number, the jet-isothermal bottom wall spacing, and the distance between two jets on heat transfer and flow field was examined. Air was chosen as the working fluid (Pr = 0.71). It is found that multi-cellular flow is formed in the impingement region due to interaction between two jets and entrainment effects in the duct. The mean Nusselt number increases almost linearly with increasing of Reynolds number at isothermal surface. When Reynolds number of the first jet is higher than second one the heat transfer is enhanced significantly.     

Periodic heat transfer in directly opposed free and forced convection flow

An experimental study of the heat transfer from small circular cylinders placed horizontal to a downward flowing air stream is reported. Based on heat-transfer measurements and flow visualization, a model for directly opposed free and forced convection was developed. Three modes of flow were observed. For very low velocities the free convection, buoyant plume dominates the heat transfer. At a “lower critical” Reynolds number, when the free and forced convections are of the same order of magnitude, a well defined periodic heat transfer was obtained. The periodic heat transfer was due to the build-up of the buoyant forces to a magnitude where they overcame the downward force of the air flow. At an “upper critical” Reynolds number the periodic heat transfer abruptly ceases. For velocities greater than the upper critical limit the forces due to the air flow dominate. A potential like, laminar sheet forms, as a shroud around the thermal layer of the hot cylinder. The average heat transfer from the cylinder decreases with increasing Reynolds number for both the case of dominant free convection and the periodic heat-transfer regime. The minimum value of the heat transfer occurred at the upper critical Reynolds number.     

Effect of thermal boundary condition on forced convection from circular cylinders

Researchers commonly assume that the effect of constant temperature and constant heat flux thermal boundary condition on forced convection from a circular cylinder is negligible. Such claim was assessed using Computational Fluid Dynamics for Reynolds number, Re, ≤ 3,000. The flow was directly solved without modeling for Re?≤?300, and Large Eddy Simulation was used for Re?=?1,000 and Re?=?3,000. Constant property simulations were done for Prandtl number of 0.7. Fluid dynamics parameters: drag coefficient, Strouhal number, separation angle and recirculation length, were used to assess the accuracy of the simulations. The main parameter of interest, the Nusselt number, is higher locally for constant heat flux than constant temperature for 60?≤?Re?≤?3,000. In addition, the ratio of overall Nusselt number for constant heat flux to that for constant temperature increases for Re < 30 and is ～1.15 between 30?≤?Re?≤?3,000, showing that thermal boundary condition has significant effect on heat transfer from circular cylinder. Finally, the study developed empirical correlations relating overall Nusselt number to studied Reynolds number range.     

Heat and fluid flow across a rotating cylinder dissipating uniform heat flux in 2D laminar flow regime

Free-stream flow and forced convection heat transfer across a rotating cylinder, dissipating uniform heat flux, are investigated numerically for Reynolds numbers of 20–160 and a Prandtl number of 0.7. The non-dimensional rotational velocity (α) is varied from 0 to 6. Finite volume based transient heatline formulation is proposed. For Re = 100, the reasons for the onset/suppression of vortex shedding at a critical rotational velocity is investigated using vorticity dynamics. At higher rotational velocity, the Nusselt number is almost independent of Reynolds number and thermal boundary conditions. Finally, a heat transfer correlation is proposed in the 2D laminar flow regime. Cylinder rotation is an efficient Nusselt number reduction or cylinder-surface temperature enhancement technique.     

Effect of thermal buoyancy on vortex shedding past a circular cylinder in cross-flow at low Reynolds numbers

This paper demonstrates the vortex shedding process behind a heated cylinder in a cross-flow at low Reynolds numbers under the influence of thermal buoyancy. The simulations were performed using an SUPG-based finite element technique. The range of Reynolds numbers was chosen to be 10–45. The flow was steady in the absence of thermal buoyancy. The eddy length and the separation angle were computed for the steady separated flow in the above range of Reynolds numbers. The results were in agreement with those reported in the literature. The Nusselt number distribution around the heated cylinder was also computed in the above range of Reynolds numbers for forced convective flows. The results compared fairly well with available experimental results. The effect of superimposed thermal buoyancy in the same range of Reynolds numbers was studied for various Richardson numbers. The steady separated flows become unsteady periodic in the presence of superimposed thermal buoyancy. For the unsteady periodic flows, the Strouhal numbers were computed. The separation angles and average Nusselt number for such unsteady flows were found to vary with time.     

Nonlinear flow and heat transfer dynamics of a slot jet impinging on a slightly curved concave surface

The nonlinear flow and heat transfer characteristics for a slot jet impinging on a slightly curved concave surface are experimentally studied here. The effects of jet Reynolds number on the jet velocity distribution and circumferential Nusselt numbers are examined. The nozzle geometry is a rectangular slot and the dimensionless nozzle-to-surface distance equals to L^⁎ = 8. The constant heat fluxes are accordingly applied to the surface to obtain an impingement cooling by the air jet at ambient temperature. The measurements are made for the jet Reynolds numbers of 8617, 13 350 and 15 415. New correlations for local, stagnation point, and average Nusselt numbers as a function of jet Reynolds number and dimensionless circumferential distance are reported.     

Heat transfer from a circular cylinder to mixtures of water and ethylene glycol

Local heat transfer by forced convection from a circular cylinder in crossflow is investigated for Reynolds number from 2 × 10³ to 9 × 10⁴ and Prandtl number from 7 to 176. The working fluids are water and mixtures of ethylene glycol and water. The cylinder is uniformly heated by passing a direct electric current through a thin surface heater. The influence of Reynolds number and Prandtl number on the distributions of local Nusselt number around a circular cylinder in crossflow is described.     

Axisymmetric impinging jet excited by a synthetic jet system

A controlled impinging jet is a promising tool for various heat/mass transfer applications, such as drying technologies or cooling of highly loaded electronic devices or gas turbine blades. An axisymmetric air jet was excited using a system of four synthetic jets distributed around the circumference of the primary nozzle. First, the control synthetic jets were measured alone. After an adjustment, the primary axisymmetric jet was excited to the helical or bifurcating modes, and its behavior was studied experimentally including an impingement effect to the wall. For comparison purposes, a reference steady (unforced) jet from the same nozzle was also measured. The flow visualization, hot-wire anemometry, PIV, and naphthalene sublimation techniques were used. The main purpose was to investigate the influence of the actuation on the impingement heat transfer at the Reynolds numbers 1600 and 5000.The effects of the Strouhal number and nozzle-to-wall spacing on a distribution of the local heat transfer were evaluated. The most significant effects were found at the Strouhal numbers 0.14–0.32 at the ratio of the control to primary jet momentum rates only 0.24–2.4%. Under small nozzle-to-wall spacing H/D = 2, the excitation led to heat transfer increase in the stagnation area – the most prominent enhancement 40% was found at the stagnation point. Under moderate nozzle-to-wall spacing H/D = 6, the excitation made more uniform the Nusselt number distribution by means of a substantial reduction of the stagnation heat transfer rate.     

Numerical study of mixed convection nanofluid in an annulus enclosure between outer rotating cylinder and inner corrugation cylinder

Numerical investigations are presented for mixed convection problems in a concentric inner sinusoidal cylinder and an outer rotating circular cylinder, which were kept at constant hot and cold temperatures, respectively. The free space between the cylinders and the enclosure walls was filled with a water‐Cu nanofluid. The governing equations are formulated for velocity, pressure, and temperature formulation and are modeled in COMSOL5.2a, a partial differential equation solver based on the Galerkin finite element method. The governing parameters considered are the solid volume fraction, [0, 0.02, 0.04, and 0.06], Re (1, 25, 100, 200, and 300), and Ra (less than 10⁴), and the inner cylinder corrugation frequencies varied from (N = 3, 6, and 9). According to the calculations, the Reynolds number, the Rayleigh number, the nanoparticle volume fraction, and the number of corrugations play an important role of forming the stream and isothermal lines, the local and the average Nusselt number inside the annulus enclosure. The average Nusselt number decreases with increasing Reynolds number and the number of corrugations, while it increases as the Rayleigh number and the volume fraction increase.     

HEAT AND FLUID FLOW ACROSS A SQUARE CYLINDER IN THE TWO-DIMENSIONAL LAMINAR FLOW REGIME

The flow structure and heat transfer characteristics of an isolated square cylinder in cross flow are investigated numerically for both steady and unsteady periodic laminar flow in the two-dimensional regime, for Reynolds numbers of 1 to 160 and a Prandtl number of 0.7. The effect of vortex shedding on the isotherm patterns and heat transfer from the cylinder is discussed. Heat transfer correlations between Nusselt number and Reynolds number are presented for uniform heat flux and constant cylinder temperature boundary conditions.     

Forced convective heat transfer with impinging slot jets of meso-scale

Measurements were made to investigate the localized heat transfer behavior of submerged slot jets. The experiments were performed with kerosene jets impinging on a vertical constant-heat-flux surface from a meso-scale slot nozzle 125 μm in width with Re = 600–1200 and nozzle-to-plate spacing Z/B = 2–20. Heat transfer coefficients at the stagnation line were measured and correlated as a function of jet Reynolds numbers and Prandtl numbers. Lateral distributions of local heat transfer coefficients were also determined and correlated. Non-monotonic variations and unusual behavior of local heat transfers were observed and attributed to the possible transition from a laminar to a turbulent flow. This transition takes place within an extremely short distance of 400–500 μm.