Average Surface Heat Transfer Coefficient Research Articles

Study on the effect law of jet angle on the cooling of hot-rolled L-beam

The uniformity of cooling affects the organizational properties and product quality of the L-beam. Problems such as bending deformation of the L-beam and excessive residual stresses inside the finished product are mainly caused by uneven cooling. The jet angle is an important cooling parameter in the cooling process of L-beam. In this study, the flow field and cooling law during jet cooling of hot-rolled L-beams were investigated by means of numerical simulation using the jet angle as a variable. The jet angle is −45°∼30° on the lower surface of the short edge of the L-beam due to space limitation, and the jet angle is −45°∼45° on the rest of the parts, and the other process parameters are kept unchanged. The velocity distribution, pressure, water flow, and temperature distribution on the cooling surface and the average heat transfer coefficient were analyzed. The results show that the jet angle has a significant effect on the cooling of each part of the L-beam. With the increase of the jet angle, the asymmetry of water velocity, pressure distribution, water flow distribution, and temperature distribution on the surface of the cooling surface increased significantly. The distribution of heat flux corresponds well to the distribution of temperature. As the jet angle increases the surface average heat transfer coefficient gradually decreases, and the maximum average heat transfer coefficient appears in the vertical jet. Gravity will also have a certain effect on the cooling of the L-beam.

Effects of cross-sectional shape on flow and heat transfer of the laminar flow of supercritical carbon dioxide inside horizontal microchannels

Supercritical carbon dioxide (sCO2) flow at the micro scale provides an excellent heat transfer solutions for a range of microelectronics cooling applications. The cross-sectional shape of the microchannel (in combination with the abruptly changing thermophysical properties of sCO2) affects the heat transfer and flow characteristics, but these effects are not well understood at constant wall temperatures as most of the previous studies were performed at constant surface heat fluxes. Current study numerically investigated, using CFD, the effects of different cross-sectional shapes, such as circle, equilateral triangle, semi-circle, and square, in zero gravity (space) and in terrestrial conditions, at constant wall temperatures.In all cases, the hydraulic diameter of the microchannel was 100 μm, the total length was 27 mm, and the heated length was 25 mm. A total of 216 cases were analyzed to predict the effects of surface temperature ranging from 310 to 330 K, pressures ranging from 8 to 10 MPa, and Reynolds numbers from 100 to 500 on the heat transfer and flow. It was revealed that the microchannel with triangular cross-section has the highest heat transfer rate, highest mass flow rate, and lowest pressure drop both in zero gravity and in terrestrial conditions. The total surface heat transfer rate for the triangular microchannel was about 49% higher than the circular microchannel at a pressure of 8 MPa, around 53% higher at 9 MPa, and about 55% higher at 10 MPa. However, the average surface heat transfer coefficient displayed a complex behavior, and it depends on the combination of surface temperature, Reynolds number, and operating pressure. The surface heat transfer rate and pressure drop were increased for all shapes at terrestrial conditions, emphasizing that the effects of buoyancy cannot be neglected. Overall, this study revealed a complex behavior of the flow of sCO2 inside microchannels of different cross-sectional shapes, and the triangular cross-section performed better than other shapes.

Heat Transfer Investigation during Condensation on the Horizontal Pipe

This paper presents an experimental investigation of condensation heat transfer by gradient heatmetry. The experiments were carried out during the condensation of saturated steam at atmospheric pressure on the cooled surface of a horizontal pipe. The distributions of the local heat flux, surface temperature, and heat transfer coefficient along the circumference of the horizontal pipe were experimentally determined. The surface average condensation heat flux on the horizontal pipe was about 141.06 kW/m2. The proposed method allows us to determine the area of condensate accumulation on the pipe (in the range of azimuth angle φ = 150…180∘) in which the heat flux decreases by 34% of the average value. The heat flux per unit area relative uncertainty was about 5.2%. The surface-averaged heat transfer coefficient during condensation on the horizontal pipe was about 5.5 kW/(m2×K), and relative uncertainty was about 9.4%.

Numerical analysis of nucleate boiling in rolling rod bundle with tilted axis using two-fluid modeling approach

In this study, an Eulerian-Eulerian two-fluid approach was applied to simulate nucleate boiling in a vertical rolling 2 × 2 rod bundle with a tilted axis. For this purpose, the standard multiphaseEulerFoam solver of OpenFOAM was modified to include the fictitious forces necessary in a non-inertial frame of the reference formulation. High-performance computing was then applied to analyze the effects of oscillation on boiling and the physics of the fluids inside the rod bundle. In particular, the effect of a tilted rolling axis was investigated for the first time. The results indicated that the space-averaged void fraction and surface-averaged heat transfer coefficient decrease in the rolling cases compared with the stationary case. Furthermore, it was found that while the tilt angle has a negligible effect on the space-averaged values, it may have a significant impact on the maximum value of the void fraction. The higher the tilt of the rolling axis, the higher the variation in the maximum field values. The results of this study will further improve our understanding of wall nucleate boiling, which is critical for the design, development, and safety of light water-cooled reactors (LWR).

Enhancing Jet Array Heat Transfer: Review of Geometric Features of Nozzle and Target Plates

Adequate cooling of electronic components and packages is an ever-increasing challenge as devices become more powerful, compact, and portable. Highly integrated and high-performance solutions are sought across a wide range of industrial and commercial sectors, requiring new methods and means of cooling beyond the traditional fan-fin heat sink. Impinging jets, when arranged in arrays, can achieve very high local and surface-averaged heat transfer coefficients whilst providing sufficient surface area coverage for a broad range of electronic devices. As such, they are a high-potential candidate for next generation heat exchangers for cooling high heat flux electronics. This review article focusses primarily on the means by which the heat transfer of impinging jet arrays can be enhanced by straight-forward alterations of the geometric features of the jet orifices, nozzle plate, and impingement surface, as compared with the baseline case of round jets impinging on flat surfaces. From non-circular jets, to patterned micro-roughness, the review aims to collate and synthesize the state-of-the-art, highlighting advantages and pitfalls, to aid thermal engineers in designing high performance jet impingement heat exchangers.

Influence of heat source distribution optimization on gallium heat transfer characteristics in a annular tube based on MLA and CFD

ABSTRACT In this study, innovative combined Machine Learning Algorithm (MLA), Genetic Algorithm (GA) and Computational Fluid Dynamics (CFD) was used to investigate the influence of nonuniform heat source distribution on gallium heat transfer performance in the annular tube, and an optimization scheme of heat source distribution was proposed. First, the heat source distribution scheme was quantified using four intervals A, B, C, and D from the inlet to the outlet, and the effect of different schemes on the heat transfer performance was investigated by CFD. Second, the prediction model of the average surface heat transfer coefficient under different heat source distributions was established based on various MLAs, and the effect of heat source interval on heat transfer performance was obtained through the model analysis. The results show that the intervals A and B have more influence on heat transfer performance, the heat transfer coefficient increases with the increase in A and B intervals. Finally, the optimal heat source distribution scheme is solved by GA. Compared with the traditional distribution scheme, the optimal scheme makes the inner wall temperature more uniform, reduces the high-temperature areas, reduces the maximum temperature by 2.6%, and increases the average surface heat transfer coefficient of the inner wall by 6.42%.

Optimization of tube arrangement and phase change material for enhanced performance of solar air heater- A numerical analysis

Application of Solar Air Heater (SAH) could be extended to off sunshine hours by integrating SAH with a Thermal Energy Storage System (TESS). Design optimization of TESS will enhance thermal performance of SAH integrated with TESS. Mass flow rate and inlet temperature of air, type of Phase Change Material (PCM), alignment and dimensions of tubes in TESS play an important role in its design optimization. In the present study, numerical analysis using ANSYS FLUENT was carried out to optimize the design of TESS. Mass flow rate of air in terms of velocity at the inlet was changed from 3.5 m/s to 5.5 m/s in a step of 0.5 m/s. Three different PCMs including paraffin wax, n-octadecane, calciumchloride hexahydrate (CaCl2•6H2O) were considered. Inline, staggered and circular arrangement of tubes were considered in the TESS. Temperature of air at inlet to TESS is changed to 40 °C, 50 °C, 60 °C, 75 °C, 100 °C and 125 °C. Results are obtained in terms of average tube wall temperature, average PCM temperature in tubes, average surface heat transfer coefficient and average heat absorbed by PCM. Tubes in staggered arrangement increased heat absorbed by PCM by 10.45% and 48.03% respectively with paraffin wax and CaCl2•6H2O as PCM compared to tubes in inline arrangement. There is insignificant improvement (0.09%) between staggered and inline grid arrangements when n-octadecane was used as PCM. Tubes in circular arrangement increased heat absorbed by PCM by 9.17%, 1.55% and 1.08% respectively with paraffin wax, n-octadecane and CaCl2•6H2O as PCM compared to tubes in staggered arrangement. Similarly, it increased heat absorbed by PCM by 17.89%, 1.63% and 48.59% respectively compared to tubes in inline arrangement.

Research on Heat Transfer Enhancement of Vortex-Induced Vibration of Side-by-Side Double Tubes

The fluid-induced vibration heat transfer enhancement technology has great potential, because it can effectively improve the heat transfer efficiency without consuming additional energy. A reasonable design of the elastic heat exchange tube bundle structure is the key to the engineering application of this technology. In this paper, the effects of flow velocity and the center distance on the characteristics of vortex- induced vibration heat transfer in side-by-side double circular tubes are studied. We demonstrate that, when the center distance is no more than (outer diameter), the interference effect of the flowfield around the two tubes is greater, and the amplitude of the double tube is small. However, with the increase of , the flowfield interference decreases. Within the scope of this paper, two modes of vibration are obtained: in-phase vibration and out-of-phase vibration. When and , the fluid vortex-induced elastic supported side-by-side double circular tubes’ vibration can enhance heat transfer. Especially, when , the heat transfer enhancement effect is the strongest, reaching 14.5%. However, when , the heat transfer enhancement effect is not obvious. The research on the heat transfer characteristics of fluid flowing around the static double tube shows that when the distance is small, the flow velocity around the static tubes is larger, resulting in a higher surface average heat transfer coefficient, and as the distance increases, the heat transfer coefficient decreases.

Influences of accelerating states on supercritical n-decane heat transfer in a horizontal tube applied for scramjet engine cooling

Supercritical n-decane is applied in the regenerative cooling system of scramjet engines while scramjets are always in various irregular movements. With the desire to explore heat transfer performance of supercritical n-decane under actual flight conditions, numerical simulations and analysis of different states during the accelerated flight process are carried out under three different heat fluxes and eight varying acceleration states. Specific flow and heat transfer mechanisms are explored by analyzing the temperature and flow fields affected by acceleration. Results show that acceleration weakens the intensity of the first abnormal heat transfer state but does not affect its appearance location, and there are almost no effects on the second heat transfer state. Another important conclusion is that the external factors have a great influence on supercritical n-decane heat transfer. The overall average surface heat transfer coefficient could be even up to 27.5% higher than for an ordinary horizontal tube. It is indicated that more attention should be paid to the external factors when investigating supercritical flow and heat transfer in the research and development of scramjet engine cooling.

Оптимізація теплообміну робочих поверхонь на основі локального контролю теплофізичних параметрів

The progressive trend of increasing the efficiency and operational reliability of equipment requires continuous improvement of methods for monitoring and managing work processes. One of the most promising methods for studying transport processes occurring in difficult conditions is physical modeling. The object of this research is heat-exchange prismatic surfaces, which are typical for many technical applications. The purpose of the work is to develop methods for optimizing heat transfer of working surfaces based on local control of thermophysical parameters in characteristic zones of the working environment. The studies were carried out in the ADS-1 aerodynamic stand using the methods of heat and mass analogy and hot-wire anemometry. The arrangement included four rows of prismatic elements. The essence of the local approach is to determine the average surface heat transfer coefficient from the local velocity, measured above each prismatic element, which makes it possible to estimate the spatial temperature heterogeneity of a specific arrangement and take measures to change the temperature in the desired direction by maneuvering the location of the elements. The proposed approach is fundamentally different from another widespread approach, which we called the channel one, in which information about the velocity and temperature fields is ignored, and the influence of the configuration and size of elements and their location on the hydrodynamic structure of the flow is insufficiently taken into account. The local approach allows, firstly, to more accurately diagnose the types of flow in the boundary layer of a streamlined element, determining laminar, turbulent, pseudolaminar, quasiturbulent, transition and separated flow regimes. Second, on the basis of a set of statistical data, make the transition to an arbitrarily specified arrangement of elements and thereby increase the accuracy of determining the temperature state of individual prismatic elements and the entire arrangement as a whole. Equations of similarity are proposed for calculating the local heat transfer of each face of the prism separately and the average surface heat transfer depending on the geometric and operating parameters. On the basis of the recommendations received, the thermal state of the elements of a specific arrangement was evaluated and measures were developed to improve it through targeted rearrangements of the elements.

Analysis of Multiphase Heat Transfer of TA2/Q235B Clad Plate Subjected to Impinging Liquid Jet Cooling

Three-dimensional numerical simulations are conducted to calculate the surface heat transfer coefficient (HTC) of the water jet impinging on a TA2/Q235B clad plate. The accuracy of the simulation model is validated by experiment. The simulation results are basically in agreement with the experimental results. The effect of clad plate temperature and thickness, jet height, orifice diameter, and orifice velocity on the HTC is investigated. The numerical results show that the average surface HTC of TA2/Q235B clad plate is greatly affected by plate temperature, Reynolds number (orifice velocity and orifice diameter), and plate thickness, while it is less affected by jet height. The surface HTC decreases with the increase of clad plate temperature and thickness. The surface HTC increases dramatically with the increase of jet Reynolds number. At the same Reynolds number (more than 14500), the cooling effect of increasing the orifice velocity is stronger than increasing the orifice diameter, while at the same Reynolds number (less than 14500), the orifice diameter has the larger impact on the surface HTC than the orifice velocity.

Numerical and Experimental Investigation of heat transfer and flow structures around three heated spheres in tandem arrangement

The objective of this work is to evaluate the convective heat transfer and flow characteristics around three heated spheres in a tandem arrangement. Numerical simulation and Experimental verification were performed using stationary copper spheres located inside a cylindrical channel with constant channel-to-sphere diameter ratio. Numerical simulation is done for three-dimensional steady-state flow using ANSYS-FLUENT by solving the Reynolds-Averaged Navier Stokes (RANS) equations. Over the test range of Reynolds numbers (2500-55000), the numerical results of the average surface temperature and heat transfer coefficient obtained are a reasonably good agreement with those obtained by experimental measurements. The distributions of the heat transfer coefficient, temperature profiles, velocity field and pressure coefficient around the sphere’s surface are calculated and analyzed.

Thermal characteristics in a spiral-tube buried in sand/bentonite/graphite under cooling and heating modes

ABSTRACT Thermal characteristics in a vertical spiral-tube ground heat exchanger were addressed. The influences of backfill materials (sand/bentonite/graphite blend) and operation modes were examined. In the heating and cooling modes with Reynolds number of 2500, the integrated evaluation factors of tube in blend rose all by about 20% compared with in sand. The longer recovery time was helpful to improve the heat transfer of ground tube. In cooling mode, the surface heat transfer coefficients, ranged from 225 to 610 W m−2 K−1. The average surface heat transfer coefficient under heating mode was 11 percent higher than that in cooling mode.

Control of boundary layer by ionic wind for heat transfer

In this study, local convection heat transfer technology was investigated using ionic wind. The characteristics of ionic wind using wire and plate electrodes are studied by experiment and numerical method. A particle image velocimetry (PIV) test was conducted in a wind tunnel to observe the flow behavior by the ionic wind on the heated surface. A new boundary layer was formed on the heated surface which was controlled by the ionic wind, regardless of the Reynolds number of bulk flow. The surface averaged heat transfer coefficient with the ionic wind increased by 11% at low Reynolds number (100–200) and decreased by 19% at high Reynolds number (2500–3500). It was also found that the zigzag type of wire arrangement enhanced the heat transfer coefficient by 40% compared with the single wire. It is thus concluded that ionic wind can be used for enhancing the convection heat transfer rate or insulating the local surface according to its operating condition.

A CFD modeling investigation for structure optimization of a rotary steam tube dryer

This paper presents an optimized structure to improve the performance of rotary steam tube dryer by assembling the so called ‘Y’ shape flights. A verified CFD model, which was developed and validated in the previous work, was employed to verify the optimized structure by the comparisons of concentration distribution, velocity distribution and heat transfer coefficient. The simulation results showed that assembling ‘Y’ shape flights in a rotary steam tube dryer is an effective approach to increase material filling ratio and heat exchanging area. The optimized construction is able to reduce the wear of the material to the tube surface, due to the low maximum particle velocity. The optimized structure yields a uniform particle velocity distribution and high average surface heat transfer coefficient of tubes which is about ten times of that without flights.

EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER IN INTERFIN CHANNELS OF PUNCHED SPIRAL TUBE FINNING

To reduce the metal consumption of boilers of steam-turbine power units and waste-heat boilers of gas-steam turbine units, the spiral-tape finning of low-temperature convection heating surfaces is applied. However, the heat transfer in the bundles of such tubes has not been studied sufficiently, the influence of the geometric dimensions of the cut portion of the fins has not been experimentally investigated. In most heat exchange calculation methods, based on the generalization of the results of experimental studies, this influence is also not taken into account. It is proposed to take into account the influence of the finning geometry on heat transfer by introducing correction coefficients in the generalizing equations, determined from the results of a numerical research. To evaluate the validity of the introduction of the results of numerical research into generalizing equations, an experimental investigation of heat exchange in the interfin channels of the punched spiral finning of the tubes is carried out. A research method is developed based on the results of measurements of the surface temperatures of the fins and air in different zones of the interfin channel along its height, as well as the air temperature in front of the bundle and the air flow through the bundle. The study is carried out by placing the calorimeter tubes in the transverse rows of the chess and corridor bundles of tubes at different Reynolds numbers. The results of the investigation of one of the bundle. The calorimeter is installed in the fifth row of a six-row tube bundle with fin interval of Sf = 6 mm and tube steps in the bundle S1 = 85 mm, Ś2 = 85 mm. At the Reynolds number, Red = 10042: the heat flux in the interfin channels Q = 711.0 W, the same according to electrical measurements - 731.0 W; discrepancy - 2.73 %; the heat transfer coefficient in the blade zone is αifcb = 162.0 W/m2°C; discrepancy – 2.88 %. The average surface heat transfer coefficient in the interfin channel is ͞ αifc = 132.35 W/m2°C; by numerical calculation - 122.6; discrepancy is 7.31 %. The average surface heat transfer coefficient of a single transverse row of tubes, taking into account the flow of a part of the air through the intertubular channels, is ͞ αc = 83.05 W/m2°C; the discrepancy is 5.12 %. In the remaining experiments, the difference in the values of the heat transfer coefficients is 3.19 ... 15.7 %. The results of the investigation confirm the validity of the use of the results of numerical investigation in generalizing equations for calculating the heat transfer of bundles of tubes with a spiral finning

Pressure drop and heat transfer performance in a rotating two-pass channel with staggered 45-deg ribs

A rotating two-pass ribbed square channel is applied to perform the pressure drop and heat transfer study. The inlet Reynolds number and rotation number vary from 20,000 to 60,000, and 0 to 1.02, respectively. The regional heat transfer coefficients are obtained by heated cooper plate technique. The rotational pressure drops are measured by the rotation-to-stationary pressure measurement system. The ribs with attack angle of 45-deg are staggered roughened on leading and trailing walls. Due to only one wall-temperature ratio was performed in current study, the isolated buoyancy effect was not able to be disclosed.The results show that the 45-deg ribs bring 40–80% higher heat transfer for ribbed walls and 15–65% for smooth walls. The rib-to-smooth Nu ratio declines with Reynolds number and is more sensible in the second passage. Meanwhile, 150–210% higher pressure drop in ribbed channel is observed compared to smooth one, but this penalty decreases with Reynolds number. Rotation presents more dominant influences on pressure drop in heated channel than unheated one. But rotation-to-stationary friction factors decrease with rotation number for both heated and unheated channel as Ro<0.55, after which an increase trend is observed. The presence of buoyancy effect in heated channel helps to promotes rotational pressure drop variation. Also, ribbed-to-smooth thermal performance is significantly elevated by rotation when rotation number reaches over 0.25. The surface averaged heat transfer coefficients are developed with respect to buoyancy parameters.

Experimental and numerical study on transient heat transfer for a twisted plate with different length in helium gas at various velocities

ABSTRACTThis study is aimed to investigate the transient heat transfer process between the solid surface and the coolant (helium gas) in very high temperature reactor or intermediate heat exchanger. Transient heat transfer from a twisted plate with different length in helium gas was experimentally and theoretically studied. The heat generation rate was increased with an exponential function, Q = Q0exp(t/τ), where t is time and τ is period. Experiment was carried out at various periods ranged from 35 ms to 14 s. Platinum plates were twisted with the same helical pitch of 20 mm, and the effective lengths are 26.8, 67.8 and 106.4 mm (pitch numbers of 1, 3 and 5), respectively. It was clarified that the average heat transfer coefficient approaches quasi-steady-state value when the period τ is larger than about 1 s, and it becomes higher when τ is shorter than about 1 s. The heat transfer coefficient decreases with the increase of plate length. An empirical correlation for forced convection heat transfer for a twisted plate with various lengths was obtained based on the experimental data. Moreover, numerical simulation results were obtained for average surface temperature difference, heat flux and heat transfer coefficient of the twisted plates with different length and showed reasonable agreement with experimental data. Through the numerical simulation, distribution of heat transfer coefficient on heater surface, temperature distribution and velocity distribution were clarified.

Transient thermal behavior of a microchannel heat sink with multiple impinging jets

We performed a transient numerical investigation on a microchannel heat sink with multiple impinging jets (MHSMIJ) to explore the effects on the fluid flow and heat transfer characteristics of the MHSMIJ of an unsteady impinging jet and heat flux imposed upon the substrate surface by using a computational fluid dynamics method. The heat fluxes being imposed upon the substrate surface and the inlet velocities of the jet were all set as sinusoidal functions with different amplitudes and periods with time. The effects of the amplitudes and periods of the functions on the substrate properties were analyzed. Cooling performance was evaluated by calculating the periodic average surface heat transfer coefficient, average temperature uniformity, and temperature variation of the target surfaces over a period. The results indicated that the surface heat transfer coefficient and average temperature of the cooled surface oscillated with the periodic heat fluxes, accompanied by obvious phase lags. The phase lag has a significant dependence on the periods, but little dependence on the amplitudes. The material properties of the substrate have complex influences on the transient behavior of the MHSMIJ. The periodic heat flux and periodic jet velocity significantly affected the transient thermal performance of the MHSMIJ, but had less effect on its overall performance. Further, transient heat flux and jet velocity caused non-uniform and transient temperature distributions, which will cause thermal fatigue phenomenon, and thereby have effect on the longevity of the MHSMIJ.

The dynamics of sliding air bubbles and the effects on surface heat transfer

Freely rising and sliding bubbles have been found to increase local heat transfer coefficients from adjacent heated surfaces. The latter have been exploited in various industrial applications, such as shell and tube heat exchangers and chemical reactors. Although there is a relatively large body of work on bubbles, only a very small portion of this focuses on sliding bubbles. The current study intends to expand this by understanding both the motion and surface heat transfer characteristics of sliding bubbles.Herein, results are presented on bubbles of 4.75–9.14mm equivalent spherical diameter sliding under both a heated and unheated surface, inclined at 30° relative to the horizontal. The sliding bubble path and shape oscillations are observed by a pair of high speed cameras. The frequency and amplitude of these oscillations are derived from analysis of the acquired images. Heat transfer is measured using a high speed infra-red camera synchronised to the video cameras, which is spatially and temporally aligned with the high speed images, allowing for the relationship between bubble motion and heat transfer to be observed.It has been found that bubbles in the range tested exhibit a sinusoidal motion. This motion is likely linked to the asymmetrical generation and shedding of vortices, with one vortex shed for each half period of path oscillation. It was observed that the bubble shape fluctuations were closely linked to the path oscillations and therefore vortex shedding. At higher bubble volumes, the bubble interface was found to recoil after a vortex is shed. There is little difference between bubble motion on a heated and unheated surface at lower bubble volumes, but at higher volumes a thinning of the bubble tips is observed along with a less-smooth interface, both of which are attributed to the thermal boundary layer at the surface. The bubble drag coefficient is also decreased when the surface is heated. Two-dimensional, time-varying surface heat transfer patterns reveal local heat transfer coefficient enhancements of up to 8 times that corresponding to natural convection levels, while the global surface-averaged heat transfer coefficient was up to twice that of natural convection. The mechanism of this heat transfer enhancement is a combination of forced convection by the sliding bubble and vortices shed at half the bubble path oscillation frequency that draw cool fluid from the bulk towards the surface. In the far wake, these vortices form isolated, elliptical regions of cooling that remain for many seconds after the bubble has passed. Interestingly, there are regions of the bubble wake where the heat transfer coefficient briefly drops below natural convection levels, highlighting the complexities of the fluid mechanics behind multiphase cooling.