Turbulent Heat Transfer Research Articles

Deformation and Breakup of Bubbles and Drops in Turbulence

Fragmentation of bubbles and droplets in turbulence produces a dispersed phase spanning a broad range of scales, encompassing everything from droplets in nanoemulsions to centimeter-sized bubbles entrained in breaking waves. Along with deformation, fragmentation plays a crucial role in enhancing interfacial area, with far-reaching implications across various industries, including food, pharmaceuticals, and ocean engineering. However, understanding and modeling these processes are challenging due to the complexity of anisotropic and inhomogeneous turbulence typically involved, the unknown residence time in regions with different turbulence intensities, and difficulties arising from the density and viscosity ratios. Despite these challenges, recent advances have provided new insights into the underlying physics of deformation and fragmentation in turbulence. This review summarizes existing works in various fields, highlighting key results and uncertainties, and examining the impact on turbulence modulation, drag reduction, and heat and mass transfer.

Employment of roughened absorber palate and jet nozzles with different hole shapes for performance boost of solar-air-heaters

In this study, roughened absorber plate and jet nozzle are simultaneously employed to enhance the efficiency of a solar based air heater. The impact of jet nozzle hole shape is comprehensively investigated for the mentioned system which have not been investigated before. Three-dimensional steady-state simulations of incompressible turbulent fluid flow and heat transfer are performed using the Reynolds-Averaged Navier-Stokes (RANS) equations to model the jet solar air heater (JSAH). Five different jet hole shapes (circular, triangular, square, hexagonal, and rectangular) are examined and results are compared with conventional structure. The thermal and hydraulic performance of the JSAH is assessed using a variety of geometric and operating parameters, including jet diameter ratio (dj/Dh), jet hole streamwise pitch (Lj/Dh), and Re number (Re). The results show that the ratio of (Nu/Nus) initially increases and then decreases as the hydraulic diameter of the jet holes increases. Circular jet holes consistently outperform other shapes, while rectangular and triangular jet holes perform poorly. The pressure penalty (f/fs) decreases with increasing (dj/Dh), with circular jet holes exhibiting the lowest value. Increasing the Reynolds number enhances heat transfer and pumping power but leads to a decrease in (Nu/Nus) and negatively affects the THPP of the JSAH compared to a conventional smooth solar air heater. The study also reveals that the THPP initially increases with streamwise pitch (Lj/Dh) but then declines, following the trend of (Nu/Nus). The results demonstrate up to 4.5 times higher Nusselt numbers with a jet solar air heater versus a smooth solar heater. A maximum thermal-hydraulic performance parameter of 1.64 indicates the proposed design's efficacy. This research furnishes critical insights into optimizing solar air heater performance by examining jet nozzle hole shape, streamwise pitch, and Reynolds number. It enables enhancing thermal efficiency and thermo-hydraulic performance.

A general approach for predicting convective heat transfer coefficients in turbulent systems

A generalized approach for predicting turbulent convective heat transfer coefficients using large eddy simulation is presented. This model is based on semi-empirical transport theory, which links the local convective heat transfer rate to the local energy dissipation rate within the near-wall boundary layer. We apply these approaches to agitated tanks, pipe flow systems, cylinders in crossflow, and tube bundles. We validate the generality and reliability of the theoretical model against expectations from experimentally derived empirical design correlations.

Multi-scaling analysis of turbulent boundary layers over an isothermally heated flat plate with zero pressure gradient

A meticulous investigation into turbulent boundary layers over an isothermally heated flat plate with zero pressure gradient has been conducted. Eight distinct turbulence models, including algebraic yPlus, standard k-ω, standard k-ε, length-velocity, Spalart-Allmaras, low Reynolds number k-ε, shear stress transport, and v2-f turbulence models, are carefully chosen for numerical simulation alongside thermal energy and Reynolds-Averaged Navier-Stokes equations. A comparative analysis has determined that the Spalart-Allmaras model exhibits remarkable agreement with the results from direct numerical simulation, making it a reliable tool for predicting turbulent heat transfer and fluid flow, particularly at higher Prandtl and Reynolds numbers. Subsequently, a multi-scale investigation employs a comprehensive four-layer structure scheme and encompasses various momentum thickness Reynolds numbers of 1432, 2522, and 4000, and Prandtl numbers of 0.71, 2, and 5. The subsequent investigation reveals the governing non-dimensional numbers' substantial impact on the distribution and magnitude of mean thermal and flow characteristics. Notably, the scaling of mean thermal and momentum fields discloses the existence of a meso or intermediate layer characterized by a logarithmic nature unique to itself. The multi-scaling analysis of the flow field demonstrates greater conformity with the selected scaling variables primarily relying on the Reynolds number. Furthermore, the scaling of the energy field yields compelling outcomes within the inner and intermediate layers. However, according to the four-layer theory, minor discrepancies are observed in the outer layer when using the current scaling.

An inverse optimization of turbulent flow and heat transfer for a cooling passage with hierarchically arranged ribs in turbine blades

Owing to the limited cold-air amount and pressure in supply systems, high-efficient heat transfer with low-level friction loss is highly desired for cooling units of a turbine blade. To exploit the potential improvement of hierarchically arranged ribs in cooling passages proposed previously, multi-parameter optimizations for rib arrangements are implemented by integrating the simplified conjugate-gradient algorithm with the turbulent flow and heat transfer model. Rib heights as design variables are optimized with various performance indices as objective functions at a fixed Re. The optimizations confirm that using the wall temperature difference and Nu as the objective function, respectively, a limited heat transfer improvement is achieved with a greatly increased friction loss. Taking the overall performance factor as the objective function, different optimal designs at different constraint conditions possess hierarchical characteristics. A significant friction loss reduction of 52.1%, 54.7%, and 54.8%, is achieved with a moderate heat transfer loss of 10.9%, 7.0%, and 2.3%. Despite different thermal and friction performances, their overall performances are consistent with a remarkable increase of 13.9%, 21.2%, and 27.3%. Finally, the optimization strategy coupling the multi-parameter optimization and hierarchical scheme is confirmed as effective for enhancing the thermohydraulic performance of convective heat transfer systems with perturbation elements.

Numerical and experimental investigation of laminar and turbulent convective heat transfer in a coiled flow reverser with twisted tape insert

This paper presents a comprehensive investigation of heat transfer enhancement and pressure drop in a Coiled Flow Reverser (CFR) with a twisted tape insert. Both laminar (10 < Re < 2000) and turbulent (12,000 < Re < 18,000) regimes have been studied using both experimental and numerical methods. The effects of twist ratio of twisted tape, inlet Reynolds number, and perforating the twisted tapes have been investigated. The study found that the best performance in the laminar regime is achieved with a twisted tape with a twist ratio of 15.73 and no holes on it at a Reynolds number of 2000. In the turbulent regime, the same twisted tape performs the best at a Reynolds number of 17,000. The highest Thermal Performance Ratio (TPR) in laminar regime is 1.55 and in turbulent regime is 0.99. In comparison to the straight pipe, all types of CFRs show better performance in heat transfer enhancement. Also, as the Reynolds number increases, the friction coefficient decreases in all configurations. The study reveals that the twisted tape insert enhances heat transfer performance while increasing pressure drop. This study provides valuable insights into the design and optimization of heat exchangers with CFR. The findings can be used to improve the efficiency and performance of various industrial processes, such as refrigeration systems, air conditioning, and power plants.

Discrepant trends in global land-surface and air temperatures controlled by vegetation biophysical feedbacks

Satellite-based land surface temperature (Ts) with continuous global coverage is increasingly used as a complementary measure for air temperature (Ta), yet whether they observe similar temporal trends remains unknown. Here, we systematically analyzed the trend of the difference between satellite-based Ts and station-based Ta (Ts–Ta) over 2003–2022. We found the global land warming rate inffered from Ts was on average 42.6% slower than that from Ta (Ts–Ta trend: −0.011 °C yr−1, p = 0.06) during daytime of summer. This slower Ts-based warming was attributed to recent Earth greening, which effectively cooled canopy surface through enhancing evapotranspiration and turbulent heat transfer. However, Ts showed faster warming than Ta during summer nighttime (0.015 °C yr−1, p < 0.01), winter daytime (0.0069 °C yr−1, p = 0.08) and winter nighttime (0.0042 °C yr−1, p = 0.16), when vegetation activity is limited by temperature and solar radiation. Our results indicate potential biases in assessments of atmospheric warming and the vegetation-air temperature feedbacks using satellite-observed surface temperature proxies.

Optimization of rifled tube by parametric changes using CFD for goodness factor enhancement

AbstractIn this study, extensive research was conducted using computational fluid dynamics (CFD) ANSYS®–FLUENT, with the k–ε turbulence heat transfer module to optimize the rifled‐tube heat exchanger through parametric modifications for heat transfer and Goodness factor enhancement. This is based on predicting the performance of various heat transfer surfaces such as rectangular and two opposite right‐angled triangular ribs. The objective of this research was to enhance the heat transfer efficiency of a rifled tube with rectangular ribs, a height of 0.775 mm, and helix angles of 30° and 58°. The research findings demonstrated that the heat transfer rate improved owing to the establishment of a secondary helical or swirl flow near the wall region, leading to an enhanced heat transfer. The modification of the rib geometry from rectangular to two opposite right‐angled triangles increased the heat transfer by 17%, whereas the pressure drop remained relatively constant at 0.62 mm of rib height. This benefit is directly translated into heat transfer without any change in pressure drop. The values of the friction factor (f), Nusselt number (Nu), and goodness factor are appropriate for the new geometry in relation to the reference geometry. Rifled‐tube designers can use CFD heat transfer analysis to optimize the design, reduce the need for physical prototypes, and test similar applications and rib configurations.

DNS investigation of flow and heat transfer characteristics of supercritical carbon dioxide over a backward-facing step

This study employed direct numerical simulation to investigate the turbulent flow and heat transfer characteristics of supercritical carbon dioxide (SCO2) in a vertical backward-facing step (BFS). Three cases were considered: constant property flow, SCO2 forced convection, and SCO2 mixed convection. All cases were conducted with an inlet Reynolds number of 4805, and the expansion rate of BFS is set at 1.5. Mean statistics like velocity, temperature, Reynolds stress, turbulent heat flux, and thermophysical properties are given and discussed. The results show that the turbulent and heat transfer characteristics of SCO2 over a backward-facing step are significantly different from those of conventional fluids due to the dramatic changes in its thermophysical properties near the pseudo-critical temperature. In the case of SCO2 forced convection, though the large thermophysical property variations do not have an obvious impact on the mean flow field, the near-wall mean density decreases largely and thus Reynolds stress and turbulent heat flux near the wall are reduced significantly, which in turn leads to a substantial reduction in the local skin friction coefficient and Nusselt number. In the SCO2 mixed convection, buoyancy substantially distorts the flow, resulting in a significant reduction in the size of the recirculation zone. Simultaneously, buoyancy acts as an additional driving force leading to the formation of a wall jet. These combined effects induce intensified turbulence near the heated wall, which enhances the turbulent heat flux and facilitates efficient heat transfer. The instantaneous vortical structures based on Q criterion also clearly show the intensified turbulence in the near-wall region.

The enhanced dew-point evaporative cooling with a macro-roughened structure

Dew-point evaporative cooling (DPEC) is renowned for its great cooling effectiveness and energy savings. However, to improve its cooling performance by optimizing the cooler's common design parameters (e.g., operating conditions and channel dimensions) has reached a bottleneck where further breakthroughs are difficult to make. Although macro-roughened structures have been found effective to enhance the heat transfer in many devices, the lack of fundamental studies and understanding on the enhanced heat and mass transfer limits their utilization in DPEC. Therefore, this study aims to investigate the enhanced DPEC with rib-roughened dry channels. A dew-point evaporative cooler with seven pairs of dry-wet air channels was constructed and tested under varying supply air and operating conditions, which demonstrated excellent cooling performance particularly at high air velocities. Concurrently, a novel low-Reynolds number computational fluid dynamics (CFD) model was developed to capture the turbulent heat and mass transfer mechanism, validated with a maximum discrepancy of 8% by the experimental data. An in-depth theoretical investigation of the rib-roughened channels show that detached vortices found near the ribs can enhance the convective heat transfer in the dry channel, as well as heat and mass transfer in the wet channel. The average Nusselt number in the dry and wet channels can reach 4 and 1.13 times of that in flat channels, respectively, and the average Sherwood number in the wet channel can reach 1.14 times. More enhanced cooling effect is observed under high air velocity conditions, leading to a reduction of product air temperature by 2.3–3.0 °C and an increase in dew-point effectiveness by 0.14–0.18. The fundamentally enhanced heat and mass transfer in the macro-structured channels provides a new research direction for the development of DPEC.

Large Eddy Simulation of turbulent flow and heat transfer in a Kenics static mixer

Large Eddy Simulation of turbulent flow and heat transfer in a Kenics static mixer

Thermal performance improvement of a circular tube-and-fin heat exchanger by ellipsoidal protrusions on fin surfaces

A novel fin stamped out with ellipsoidal protrusions is proposed to improve the performance of a circular tube-and-fin heat exchanger in this paper. The ellipsoidal protrusions have a similar function as the traditional plane or curved vortex generators that can generate longitudinal vortices in the flow channel. The effect of different attack angles of the ellipsoidal protrusions on the thermal performance of a circular tube-and-fin heat exchanger is numerically analyzed. The turbulent flow and heat transfer properties were investigated by Standard k-ε turbulence model with finite volume method. The results illustrated that the ellipsoidal protrusions can substantially improve the intensity of secondary flow and enhance the heat transfer. The ellipsoidal protrusions exhibited better heat transfer than the traditional vortex generators. Nu of the fin with ellipsoidal protrusions increased up to 31.73 % and 19.95 % compared with the smooth fin and the fin with vortex generators, respectively. Both Nu and f of the fin with ellipsoidal protrusions increased with the increase in attack angle. An optimum attack angle of 20° of the ellipsoidal protrusion existed for the highest thermal performance factor of 1.165. Fitted equations for Nu, f, and JF were provided for facilitating application to engineering.

Turbulent Schmidt and Prandtl Numbers in the Boundary Layer on a Wall with Film Cooling when a Foreign Gas is Injected through a Porous Insert

Variations in the turbulent Schmidt and Prandtl numbers in the boundary layer on a wall with film cooling when helium is injected into xenon flow through a porous insert is investigated numerically using the three-parameter RANS turbulence model supplemented with transport equations for turbulent heat and mass transfer. The results obtained are compared with calculated data for constant turbulent Schmidt and Prandtl numbers.

The Effect of Boundary Conditions on the Modeling of Anomalous Intensification of Turbulent Heat Transfer in an Inclined Groove in the Wall of a Narrow Channel

The effect of the boundary conditions of the first and second kind on the predicted characteristics of anomalous heat transfer intensification in the case of turbulent low-velocity air flow over an inclined groove in the hot wall of a narrow channel is numerically investigated. It is shown that the isothermality conditions lead to a considerable underestimation of the thermal efficiency of a surface structured with narrow grooves, compared with the conditions of heat flux constancy.

Wind-tunnel measurements of sensible turbulent heat fluxes over melting ice

Accelerated glacier melt and the loss of perennial snowfields have been associated with increased warming in polar regions, at rates up to four times faster than the rest of the world, thereby reinforcing the critical need for improved models (and predictions) of glacier melt. An essential requirement for such models is an improved understanding of the sensible heat fluxes over glaciers. Since their complexity makes them difficult to model, and direct measurements of sensible turbulent heat fluxes over real glaciers are both rare and impractical, the present work involves simultaneous hot-wire anemometry and cold-wire thermometry measurements of two components of velocity and temperature above a melting glacier model in a series of wind-tunnel experiments. Both single- and multi-variable statistics were used to compare the turbulent velocity field measured over melting ice with that of a similar flow in the absence of ice. The results demonstrate that the ice's presence reduces the magnitude of the Reynolds stresses and vertical velocity variance, but also increases the streamwise velocity variance. The transient evolution of temperature statistics throughout the melt process was also investigated and found to be similar when suitably non-dimensionalized. The velocity and temperature fields were furthermore evaluated at an equivalent non-dimensional time during the melt process, in which statistics of the temperature field, and joint statistics of the vertical velocity and temperature, were studied. The present work lays the foundation for future laboratory-scale replications of the flow above melting glaciers, and provides additional insight into turbulent heat transfer over melting ice.

Effect of porous density of twisted tape inserts on heat transfer performance inside a closed conduit

This study examines the impact of varying the porosity density of twisted tape inserts (TTI) on the temperature distribution, fluid velocities, heat transfer coefficients (HTC), Nusselt numbers (Nu), turbulent kinetic energy (TKE), and performance from 5000 to 12500 Reynolds numbers (Re). The entire process involved the design of TTIs and double pipe heat exchangers using SolidWorks. Subsequently, a three-dimensional fluid flow model was employed to solve equations related to energy mass, energy, and momentum within the ANSYS Fluent interfaces. The findings highlight the noteworthy impact of high porosity TTIs, which consistently reduce temperature spans, increase fluid velocities, and greatly HTC and Nu when compared to low porosity TTI, typical TTI, and plain tubes. Furthermore, high porosity TTI significantly increases TKE, indicating increased fluid turbulence and higher heat transfer efficiency, especially at Re = 12500. The assessment of PEC emphasizes the superiority of high porosity TTI, demonstrating their significant performance increase potential of over 6.44 % over low porosity TTI and a staggering 62.5 % above typical TTI. In conclusion, high porosity TTI emerges as a potential solution for improving heat transfer efficiency and overall system performance in a variety of industrial applications, promising enhanced energy efficiency and superior performance.

Anisotropy and relaminarisation of the turbulent flow near a rotating cylindrical cavity wall

AbstractRotor internal cooling is a new concept for high power density electric vehicle drives. A turbulent pipe flow is injected into a cylindrical cavity in the rotor shaft of the motor. The flow is deflected in the cavity, accelerated in circumferential direction by the rotor wall and exits through an annular duct with the outer wall rotating. Due to the opposing effects of rotation on turbulence, a complex transitional flow develops. The strong shear layer in the jet region causes high turbulence production. On the other hand, these fluctuations are damped by the centrifugal forces due to the flow rotation. To investigate the influences of the rotation on the turbulence properties and the mean flow, highly resolved large eddy simulations are performed. It is shown that the turbulence production and attenuation due to rotation affect different components of the Reynolds shear stress tensor. This results in highly anisotropic turbulence. In certain areas, where the turbulence attenuation is strongest, the flow even relaminarises. Since the cooling efficiency depends on the turbulent heat transfer of the flow, the local turbulence characteristics are key quantities for the cooling application.

The influence of geometrical and operational parameters on thermofluid performance of discontinuous colonial self‐swirl‐inducing baffle plate in a tubular heat exchanger

AbstractAn exploratory investigation was conducted to analyze a tubular heat exchanger's turbulent heat transfer characteristics using an innovative baffle plate design. The design incorporated a perforated conical baffle plate and an adjustable rectangular deflector. The strategic placement of this baffle plate at the heat exchanger inlet facilitated a swirling flow downstream. To assess the impact of this design, three baffle plates with varying pitch ratios (l/D) were positioned in the direction of the flow. The study examined the effects of pitch ratio (ranging from 0.6 to 1.2), deflector inclination angle (ranging from 30° to 50°), and Reynolds numbers (ranging from 16,500 to 29,500). Moreover, the size of the conical baffle plate (H/D) was altered (from 0.25 to 0.15). The findings highlighted the substantial influence of pitch ratio, inclination angle, and height ratio on the heat exchanger's performance. Notably, the configuration featuring an inclination angle of 50° and height ratio of 0.15 exhibited an average performance improvement of 22% compared to other height ratios, inclination angles, and pitch ratios.

Comparison study on thermo-hydraulic performance of circular and airfoil shaped wire-wrapped rod bundle

The turbulent flow behavior and heat transfer performance of wire-wrapped rod bundles have a significant impact on the thermal–hydraulic characteristics and safety of Generation IV nuclear reactors. In this study, the airfoil-shaped wire-wrapped bundle is proposed to improve the comprehensive performance of advanced reactor cores, and the thermal–hydraulic performance has been compared with the circular-shaped wire-wrapped bundle. Special attention has been paid to the temperature variation and friction pressure loss caused by the wire wraps. Specifically, the generation of the secondary flow is the main factor. The airfoil-shaped wire-wrapped bundle exhibits a half friction pressure loss, a more uniformed temperature distribution, and a reduction of the hot spot temperature compared to the circular-shaped wire-wrapped bundle. Moreover, the airfoil-shaped wire shows a 16.3% increase in Nusselt number and a 2.0% increase in friction factor compared to circular-shaped wire, resulting in an 8.7% increase in performance evaluation criteria (PEC). The airfoil-shaped wire-wrapped bundle demonstrates better comprehensive performance than the circular-shaped wire-wrapped bundle in terms of PEC.

Numerical investigation on boiling heat transfer in a rectangular channel under sub-atmospheric environment

Water evaporators are often used as heat sinks in spacecraft. The aim of this study is to investigate the heat transfer characteristics and the spatial and temporal characteristics of bubble formation in an airborne water evaporator at an absolute pressure of 7 KPa. In order to achieve this, a rectangular single-channel model based on the structure of the water evaporator is established. Since the temperature change of the glycol in the water evaporator is not obvious when it flows through the rectangular single-channel, the inner wall surface is set to be 333.15 K in order to reduce the computational cost in the simulation. In this paper, a VOF method is adopted to numerically investigate the heat transfer characteristics of the boiling in the upper pool of the rectangular channel under a low-pressure(7 kPa) environment. Good agreement was obtained by comparing the numerical results with the experimental data with the error within 6 %. The numerical results show that bubbles rise along the wall, combine with nearby small bubbles, grow and attach to the wall, and slide along the wall until they are discharged from the top of the wall. The perturbations generated by the bubbles cause the heat transfer coefficient and wall temperature to fluctuate, with the maximum fluctuation of the wall temperature being around 5 K. In addition, the effects of different rectangular channel structure sizes (0.5 mm, 1.5 mm, and 2.5 mm) on the boiling heat transfer and bubble characteristics were investigated. A channel size of 0.5 mm leads to severe bubble coalescence, which increases the wall temperature and deteriorates the heat transfer performance. When the channel spacing is increased, bubble coalescence is improved, but a channel spacing of 2.5 mm results in less bubble turbulence and weaker heat transfer performance.