Heat Transfer Fluctuations Research Articles

Unsteady double diffusive convection inside a partial porous building enclosure subjected to time-periodic temperature boundary condition

Unsteady double diffusive convection under the influence of time-periodic sidewall temperature inside a rectangular enclosure with a porous wall is performed using numerical simulation and analytical prediction. The ranges of investigated physical parameters are listed as Darcy number (10−10 ≤ Da ≤ 101), thickness of porous wall (0 ≤ d ≤ 1), thermal Rayleigh number (103 ≤ Ra ≤ 107), buoyancy ratio (−1 ≤ N ≤ 1) and oscillating frequency of time-periodic sidewall temperature (0.01 ≤ ω ≤ 10). Results indicate that rates of heat and moisture transfer are impacted by time-periodic temperature and have a sinusoidal variation with time. The heat and moisture resonance phenomenon occurs when the convection effect is sufficiently stronger. Scale analysis has been performed to obtain the correlations of overall heat and moisture transfer rates in basic steady state and the resonant frequency, where the fluctuations of heat and moisture transfer are increased. A good agreement is observed from the analytical and numerical results. Present work could be applied to determine the heat and moisture transfer rates and resonant frequency in a thermal storage system, where the sidewall temperature is varying periodically with time due to the weather condition (solar radiation) and ambient temperature during a typical day.

Attempt to Measure Boiling Heat Transfer Fluctuation through a Visible-light Transparent Heater by Infrared Thermography

Attempt to Measure Boiling Heat Transfer Fluctuation through a Visible-light Transparent Heater by Infrared Thermography

Correspondence between boiling behavior and local heat transfer fluctuation of a falling droplet

Correspondence between boiling behavior and local heat transfer fluctuation of a falling droplet

General formula for the temporal and spatial resolutions in measuring the heat transfer fluctuation on a thin-film heater formed on a window material by infrared camera

General formula for the temporal and spatial resolutions in measuring the heat transfer fluctuation on a thin-film heater formed on a window material by infrared camera

Review on condensation heat transfer and pressure fluctuation characteristics of gas-liquid two-phase jet flow

Review on condensation heat transfer and pressure fluctuation characteristics of gas-liquid two-phase jet flow

Experimental and Numerical Study of Jet Impingement Cooling for Improved Gas Turbine Blade Internal Cooling With In-Line and Staggered Nozzle Arrays

Abstract Internal cooling of gas turbine blades is performed with the combination of impingement cooling and serpentine channels. Besides gas turbine blades, the other turbine components such as turbine guide vanes, rotor disks, and combustor wall can be cooled using jet impingement cooling. This study is focused on jet impingement cooling, in order to optimize the coolant flow, and provide the maximum amount of cooling using the minimum amount of coolant. The study compares between different nozzle configurations (in-line and staggered), two different Reynold's numbers (1500 and 2000), and different stand-off distances (Z/D) both experimentally and numerically. The Z/D considered are 3, 5, and 8. In jet impingement cooling, the jet of fluid strikes perpendicular to the target surface to be cooled with high velocity to dissipate the heat. The target surface is heated up by a direct current (DC) power source. The experimental results are obtained by means of thermal image processing of the captured infra-red (IR) thermal images of the target surface. Computational fluid dynamics (CFD) analysis were employed to predict the complex heat transfer and flow phenomena, primarily the line-averaged and area-averaged Nusselt number and the cross-flow effects. In the current investigation, the flow is confined along with the nozzle plate and two parallel surfaces forming a bi-directional channel (bi-directional exit). The results show a comparison between heat transfer enhancement with in-line and staggered nozzle arrays. It is observed that the peaks of the line-averaged Nusselt number (Nu) become less as the stand-off distance (Z/D) increases. It is also observed that the fluctuations in the stagnation heat transfer are caused by the impingement of the primary vortices originating from the jet nozzle exit.

Transient heat transfer measurements on pulsating and oscillating flows in a shock tunnel

Flow field over high-speed vehicles can be changed drastically with the addition of a spike. These spiked bodies are associated with pulsation and oscillation unsteady flow phenomena on a spiked-body configuration. The flow separation leads to fluctuations in pressure and heat transfer of high amplitude and frequency. An investigation is made in this study, to measure the heat flux using platinum thin film sensors for these two unsteady flow characteristics, which would provide additional insights and may lead to better understanding of such type of flows. Experiments are carried out on a sharp-tipped spike attached to a flat face cylinder in a shock tunnel operating at Mach 5.7. Provisions are made to vary the spike length to diameter ratio (L/D), ranging from L/D = 0.75, 1, 1.25 and 1.5 for current investigation. The measured heat transfer varied from 100 W/cm2 to 10 W/cm2 for a pulsation case (L/D = 0.75) and whereas for the oscillation case (except for the corner gauges) the sensors indicate a heat transfer variation between 50 W/cm2 to 5 W/cm2. In additions, the corner gauge shows a heat transfer of around 120 W/cm2 to 10 W/cm2. This indicates the region of shock-shock interaction. The results from schlieren technique, used for visualization of the flow field is compliments the quantitative results.

Performance prediction of trailing-edge cooling system of gas turbine blade using detached eddy simulation

This study evaluates the performance of the trailing-edge (TE) cooling system in a gas turbine blade. Eddy Simulation (DES), based on the turbulence model of Spallart-Almaras (SA), was used to simulate the TE cooling system. A TE configuration with a five-row staggered pin-fin arrangement was employed as a computational domain. Three parameters, i.e., the coefficient of heat transfer on the pin-fins surface (hpin), the coefficient of discharge (CD), and the effectiveness of adiabatic film cooling were used to assess the performances. The findings denoted that the heat transfer fluctuations occurred on the surface of the pin-fins in each row. The discharge coefficient increased with the rising of the blowing ratio. The trend predicted data of adiabatic film cooling effectiveness were in good agreement with realistic discrepancies compared to other researches, mainly for higher blowing ratio. The average effectiveness along the cut-off region was to be sensitive to the blowing ratio changes, which was attributed to the structures of turbulent flow along the mixing region.

Resonant enhancement of thermomagnetic convection of paramagnetic fluid in an enclosure due to time-periodic magnetizing force

A numerical study is performed to demonstrate the thermomagnetic convection of air in a side-wall heated enclosure under constant gravity and time-periodic magnetizing force formed by current with sinusoidal waveform. Two cases have been considered: electric coil is located at the right side of the enclosure in case 1, and in case 2 the electric coil is below the enclosure. The spectral element method is applied to obtain the numerical solutions from the time-dependent Navier-Stokes equations including the magnetizing force term. The computed results show the existence of resonance, which is characterized by the maximum fluctuation of heat transfer in the interior. And the resonance phenomenon becomes more distinctive as Ra increases. Details of fluid flow and heat transfer under resonance is scrutinized by examining the evolution of oscillating components. The results indicate that case 1 exhibits advantages in using resonance to enhance the heat transfer rate, where a smaller magnetic strength is employed to simultaneously obtain larger fluctuations of fluid flow and heat transfer.

A novel approach to correlate heat transfer and pressure fluctuation in gas–solid spouted bed

Statistical analysis of pressure fluctuations in spouted beds has been used as a well-established diagnostic tool to determine bed and flow characteristics because of its smooth operation. However, in many recent and conventional applications of spouted beds such as drying, coal gasification, catalytic conversion, biomass treatment, and chemical vapor deposition, direct estimation of the heat transfer rate from the solid bed to the gas or vice versa has proven to be difficult. A variance and spectral analysis of pressure fluctuation is extended here to characterize the heat transfer phenomena in a spouted bed. In the present study, zirconia and alumina were used as the bed materials, and argon and nitrogen were used as the spouting gases. Experiments were conducted at various heating rates for different superficial gas velocities for a range of temperatures up to 300°C. Significant changes in the gas density and viscosity with different extents of heat transfer were observed to affect the momentum diffusivity and gas–particle interaction, which in turn led to local pressure fluctuations, causing the bed to behave differently. In the present work, a novel approach is proposed to establish a link between local pressure fluctuation and the extent of heat transfer in the bed. This method shows potential for correlation of the statistics of pressure fluctuation with the thermal properties of individual solids and gases. Thus, the technique can be extended to many industrial applications for the indirect estimation of the extent of heat transfer and prediction of unknown thermal properties of products in solids or gases.

Numerical assessment of wall modelling approaches in scale-resolving in-cylinder simulations

Wall modelling in internal combustion engines (ICEs) is a challenging task due to highly specific boundary layers and a dynamically changing flow environment. Recent experimental (Jainski et al., 2013, Renaud et al., 2018) and direct numerical simulation (DNS, Schmitt et al., 2015a) studies demonstrate that scaled near-wall velocity and temperature profiles in ICEs deviate considerably from the law of the wall. Utilising the DNS data, the present paper focusses on benchmarking a scale-resolving approach with a 1-D non-equilibrium wall model (HLR-WT, Keskinen et al., 2017) in ICE-like flows. Specific emphasis is put on the compression stroke using different grids and two additional wall-modelled large eddy simulation (WMLES) reference approaches. The standard wall law based WMLES-1 produces highly grid-dependent underprediction of wall fluxes, to which WMLES-2 (Plensgaard and Rutland, 2013) and HLR-WT, employing engine-targeted wall treatments, yield considerable improvement. Differences between the improved methods are noted in detailed metrics. Throughout the compression stroke, HLR-WT provides a good match to the DNS in scaled mean boundary layer profiles for both velocity and temperature. With relevance to local heat flux distribution, the characteristic impingement-ejection process observed in the DNS is qualitatively replicated with WMLES-2 and HLR-WT. The non-equilibrium formulation of the latter allows for slight improvements in terms of local heat transfer fluctuation predictions. In contrast, coarse near-wall grids appear to be detrimental for such predictions with all approaches. The study provides evidence on the potential of the HLR-WT and WMLES-2 approaches in ICE near-wall flow prediction, advocating further investigations in more realistic engine configurations.

Numerical investigation on heat transfer of the supercritical fluid upward in vertical tube with constant wall temperature

The supercritical fluid has been extensively applied in many industry applications and heat transfer characteristics of the supercritical fluid play an important role in system safety and economic design. In this paper, heat transfer characteristics and mechanism of the supercritical fluid upward in vertical tube with constant wall temperature are numerically investigated. Heat transfer fluctuation phenomenon is found at the trans-critical section where fluid experiences the pseudo-critical point. Heat transfer fluctuation takes place only when the wall temperature is higher than the pseudo-critical temperature and the bulk temperature is lower than the pseudo-critical temperature. The traditional prediction correlation can’t predict heat transfer fluctuation at the trans-critical section correctly. Heatflux fluctuation is caused by the buoyancy effect. The buoyancy effect induces the periodic flow variation and the periodic convective heat transfer variation on the radial direction, which determine heatflux fluctuation on the wall. The fluctuation amplitude of heatflux on the wall decreases along the axial direction due to the weakened buoyancy effect. Influence of operating conditions on heat transfer of the supercritical fluid is investigated. R134a is chosen as the working fluid. Operating condition includes mass flow ranging from 500 to 800 kg/(m2 s), inlet temperature ranging from 313.15 to 343.15 K, wall temperature ranging from 403.15 to 433.15 K and operating pressure ranging from 4.35 to 5.04 MPa. It is observed that heat transfer coefficient rises with mass flow, wall temperature and operating pressure increasing, but it doesn’t vary obviously with inlet temperature. Fluctuation amplitude of heat transfer coefficient decreases with mass flow, wall temperature, inlet temperature and operating pressure increasing.

A low Reynolds number flow and heat transfer topology of a cylinder in a wake

The flow and heat transfer topology of an isothermal cylinder (diameter D) in the wake of another smaller (diameter d) cylinder is numerically investigated for a spacing ratio of L/d = 5.5 and at a Reynolds number of 200. The governing Navier-Stokes and energy equations are solved using the finite volume method. The focus is given on how the size of the upstream cylinder influences the flow and heat transfer topology around the downstream cylinder when the diameter ratio d/D is varied from 0.15 to 1.0. The upstream-cylinder shear layers for d/D < 0.3 reattach on the downstream cylinder and shed vortices behind it. They for 0.3 ≤ d/D ≤ 1.0, on the other hand, shed vortices in the gap, and the wake of the downstream cylinder features a primary vortex street followed by a secondary vortex street. The primary street evolves into the secondary street through merging and isolated-shedding processes. The arrival of a gap vortex on the front surface of the downstream cylinder generates an impulse that has the greatest impact on the heat transfer. The surface-averaged heat transfer enhances when d/D is decreased from 1.0 to 0.4, and it declines for a further decrease in d/D. The enhancement in heat transfer is ascribed to an increased shear layer velocity, reduced wake-recirculation size, and enhanced recirculation strength. A novel tertiary frequency in heat transfer fluctuation is identified. The correlation between the flow and heat transfer fluctuation at the tertiary frequency is explicatively done through first Fourier transform and wavelet analysis.

Fluctuations of radiative heat exchange between two bodies

We present a theory to describe the fluctuations of nonequilibrium radiative heat transfer between two bodies both in far and near-field regime. As predicted by the blackbody theory, in far field, we show that the variance of radiative heat flux is of the same order of magnitude as its mean value. However, in near-field regime we demonstrate that the presence of surface polaritons makes this variance more than one order of magnitude larger than the mean flux. We further show that the correlation time of heat flux in this regime is comparable to the relaxation time of heat carriers in each medium. This theory could open the way to an experimental investigation of heat exchanges far from the thermal equilibrium condition.

Heat transfer from immersed tubes in a pulsating fluidized bed

Heat transfer through immersed tubes in pulsating fluidized beds was investigated computationally using an Eulerian-Eulerian approach. The effects of pulsating frequency and configuration of the immersed tubes were studied. At low pulsating frequency, more orderly bubbling behaviors were observed and these gave rise to more uniform heat transfer rates from all tubes. An interesting heat transfer behavior whereby the heat transfer coefficient distribution around an immersed tube alternated between a needle-like and a wide fan-shaped pattern was observed for the first time in this study. An increase in pulsating frequency caused the fluidized bed to exhibit fluctuating heat transfer rates. The staggered tube configuration considered in this study was observed to aid in the development of more orderly bubbling behaviors, with heat transfer coefficient of tubes in the bottom row exhibiting similar oscillation amplitudes.

Effects of the stroke length and nozzle-to-plate distance on synthetic jet impingement heat transfer

This study focuses on the combined effect of the nozzle-to-plate distance and of the stroke length on the cooling performances of impinging synthetic jets. Infrared thermography is used as temperature transducer in conjunction with the heated thin foil heat transfer sensor to measure time- and phase-averaged convective heat transfer. All the experiments have been performed at a fixed Reynolds number equal to 5250, while different values of the dimensionless stroke length (L0/D equal to 5, 10 and 20) and nozzle-to-plate distance (H/D between 2 and 10) have been considered. At high L0/D, the heat transfer behaviour resembles that of a continuous impinging jet. It is characterized by a time-averaged stagnation Nusselt number maximum between H/D equal to 4 and 6 and inner and outer ring-shaped regions of Nusselt number maximum at short H/D. These two regions are replaced by a bell-shaped distribution at higher nozzle-to-plate distances. The existence of these regions is clearly observed through the phase-averaged heat transfer measurements. At short H/D, the heat transfer evolution reveals the simultaneous presence of two outer ring-shaped regions. The external outer region is ascribed to the strong coherence of the primary vortex ring, while the internal one is mainly due to the vortex rings generated by the Kelvin–Helmholtz instability along the trailing jet shear layer. At high H/D, the internal outer-ring shaped region disappears because of the weakening of the trailing jet Kelvin–Helmholtz vortex rings. In opposition, at short L0/D, the time-averaged stagnation point Nusselt number is found to have a maximum at lower values of H/D, and no inner ring-shaped region of heat transfer maximum is observed. This region is not present, at low dimensionless stroke lengths, because of the weakness and reduced extent of the trailing jet in the flow field. Indeed, the phase-averaged measurements mainly show the heat transfer caused by the impinging primary vortex ring. Despite the weakness of the trailing jet, the outer ring-shaped region of heat transfer maximum is observed at short H/D because of the presence of a strong primary vortex ring. In addition to that, the dimensionless stroke length and nozzle-to-plate distance also affect the heat transfer fluctuations, which decrease as L0/D decreases and/or H/D increases.

Unsteady characteristics of turbulent heat transfer in a circular pipe upon sudden acceleration and deceleration of flow

The goal of the present study is to clarify the mechanism of heat transfer enhancement in a pipe flow upon sudden acceleration and deceleration. As such, we applied a technique using infrared thermography to measure the spatio-temporal heat transfer to a water pipe flow. A rectangle-shaped pulsating flow was generated by opening and closing a solenoid valve installed downstream of the test section, and the fluctuation of heat transfer was measured simultaneously with the flow properties, such as the fluctuations of the flow rate and velocity in the pipe. First, the cycle of the pulse period was set to be relatively long (T=8s, ω′=10) in order to investigate the basic characteristics of the heat transfer caused by a sudden acceleration and a deceleration of the flow. The upper and lower Reynolds numbers for the valve-open and valve-closed conditions were set to ReD=12,000 and 3000, respectively. The duty ratio, which is the ratio of the period of valve opening to the total time, was set to 50%. Upon sudden deceleration of the flow, a complicated structure with high-heat-transfer spots appeared, and the structure was maintained for some time while diffusing. This resulted in a gradual decrease in the heat transfer coefficient, although flow rate decreased rapidly. In contrast, upon sudden acceleration of the flow, the heat transfer coefficient remained low for some time due to the re-laminarization phenomenon, until flow turbulence was initiated. Based on these results, we modified the pulsation condition to explore the possibility of heat transfer enhancement. The upper and lower Reynolds numbers were set to ReD=8000 and 0, respectively. The cycle period was varied as T=3s, 6s, 12s, 24s, and 48s (ω′=16∼4) at a duty ratio of 25%. As a result, the heat transfer was enhanced more than 50% at T=6s (ω′=11) compared to that for the non-pulsating flow at the same flow rate.

Unifying quantum heat transfer in a nonequilibrium spin-boson model with full counting statistics

To study the full counting statistics of quantum heat transfer in a driven nonequilibrium spin-boson model, we develop a generalized nonequilibrium polaron-transformed Redfield equation with an auxiliary counting field. This enables us to study the impact of qubit-bath coupling ranging from weak to strong regimes. Without external modulations, we observe maximal values of both steady state heat flux and noise power at moderate coupling regimes, below which we find those two transport quantities are enhanced by the finite qubit energy bias. With external modulations, the geometric-phase-induced heat flux shows monotonic decrease as increasing the qubit-bath coupling at zero qubit energy bias (without bias). While under finite qubit energy bias (with bias), the geometric-phase-induced heat flux exhibits an interesting reversal behavior in strong coupling regime. Our results unify the seemingly contradictory results in weak and strong qubit-bath coupling regimes, and provide detailed dissections for the quantum fluctuation of nonequilibrium heat transfer.

Construction of 2D-3C PIV and high-speed infrared thermography combined system for simultaneous measurement of flow and thermal fluctuations over a backward facing step

In the present study, simultaneous measurement of flow and heat transfer fluctuations downstream of a backward facing step in the near-wall region is presented. The instantaneous flow and convective heat transfer profiles were experimentally measured using two-dimension-three component (2D-3C) PIV and a high-speed infrared thermography (IRT) combined system at the same time. Using this measurement system, the unsteady flow behaviour in the vicinity of the heated wall and the heat transfer on the heated wall were investigated for the Reynolds number range from 2.5×103 to 5.2×103, which is based on the step height. The results showed that the forward and downwash flows appear to enhance the heat transfer in the near-wall region. It is confirmed that the simultaneous measurement of flow and heat transfer fluctuations is possible to be obtained by using this system.

Heat transfer fluctuation in a pipe caused by axially non-uniform heat distribution

This paper numerically investigates the impact of non-uniform boundary heat fluxes on the local and global Nusselt numbers of a circular pipe in fully developed turbulent flow regime. The numerical study is carried out basing on a validated three-dimensional CFD model under a series of prescribed sinusoidal wall heat fluxes. Results show that local Nusselt number fluctuation is remarkable under the heat flux with large non-uniformity and the local fluctuation ratio relatives to the uniform boundary condition is within the range of −26% to +15%. The phase-difference between heat flux profile and Nusselt number profile is expounded via mathematical treatment. To explore the underlying mechanism of the foregoing phenomenon, the analytical tool of field synergy principle is employed and is executed in a more rigorous way. It indicates that the local heat transfer fluctuation attributes to the local synergy-angle change in the very near-wall region. Besides, the average Nusselt number is also investigated. It is shown that, for a fixed amount of total heat flux, the average Nusselt number slightly decreases when the boundary heat flux changes into non-uniform, which dues to the global enlargement of synergy angle as well as the global augmentation of entropy generation increment.