Heat Transfer Enhancement In Flow Research Articles

Chaotic Behavior and Heat Transfer Analysis of Pulsating Flows with Different Frequencies

Due to the rapid development of electronic devices, the improvement of air–cooling methods for thermal management is urgent. In the present work, a promising way of coupling pulsating flow and vortex generators is proposed to eliminate the impacts of severe local high-temperature regions behind the vortex generators. Different from the traditional qualitative analysis, chaos is introduced to identify the quantitative impacts of the nonlinear dynamics of the coupling system. Particularly, the flow field was examined using the line stretching and the Poincaré map, and the chaotic characteristics of high-frequency pulsating flow were investigated. The results show that high-frequency pulsating flow enhances heat transfer mainly by promoting chaotic mixing. The chaotic fluid repeatedly stretches and folds, which increases the line stretching. Thus, the line stretching distribution can represent the heat transfer. Compared to the steady flow case, at Reynolds number of 1024 and pulsation frequency of 250 Hz, pulsating flow improves the Nusselt number by 5.82% and reduces friction factor by 2.62%, resulting in an improvement of thermal performance factor by 6.76%.

Numerical simulation of JAG-type corrugated plate structure optimization and enhanced heat transfer by pulsating flow

The paper presents a numerical investigation of the structural optimization of a JAG-type corrugated plate and enhanced heat transfer under pulsating flow based on the research results that have been achieved. Firstly, the corrugation pitch, corrugation depth, and corrugation inclination of the corrugated plate were optimized using Taguchi method to obtain the best heat transfer performance. Secondly, a detailed study is carried out on the flow and heat transfer characteristics under different pulsation velocities (0.01 m/s∼0.1 m/s), frequencies (0.4Hz–3.2Hz), and amplitudes (0.1–0.9). Additionally, the relationship between the flow variation at the contact tail of the corrugated channel and enhanced heat transfer is elucidated in combination with the field synergy theory. The results demonstrate that the application of pulsation characteristics to fluid flow enhances heat transfer efficiency under different working conditions, with significantly superior heat transfer performance observed at low flow rates compared to high flow rates. Moreover, the modification of amplitude has a greater impact on heat transfer performance compared to changes in frequency and speed conditions. Changing the amplitude can produce a better synergistic effect. The research results can provide important reference for the engineering application of enhanced heat transfer by pulsating flow.

Determination of critical thickness of porous layer for passive heat transfer enhancement for flow over 2-D cylinder

This study focuses on flow and heat transfer around a circular cylinder having porous surface. The Reynolds number (Re) is constant at Re = 100 and Darcy number (Da) is varied from 10– 6 to 10– 2 for porous layer thickness d = 0.2 R − 0.5 R . While various parametric studies have been carried out to study this geometry, no study has provided a critical thickness of porous layer for the range of (Da) studied herein. The specific focus of the parametric nature of the study is novel in that it approaches the problem from the perspective of flow and heat transfer variables to determine the critical thickness of porous layer, as opposed to viewing it as a whole. This differs from previous studies in that the problem is fundamentally limited to this simplistic system without adding many interconnected variables that will further increase complexity. The dependency of flow and heat transfer modification on various physical parameters such as the porous layer thickness, upstream and downstream lengths is investigated in detail, and a critical thickness (which optimizes the fluid flow and heat transfer characteristics) h = 0.5 R is determined. Finally, the heat transfer characteristics are analyzed to study the heat transfer behavior of the porous layer, and to determine the critical thickness of porous layer for efficient heat transfer.

Heat transfer enhancement for flow boiling of zeotropic mixtures with regulating liquid mass transfer resistance

The mass transfer resistance is a significant element leading to heat transfer degradation of zeotropic mixtures in the flow boiling. In this paper, the heat transfer enhancement of zeotropic mixtures in the flow boiling was investigated through regulating the mass transfer resistance by constructing reasonable surface structure. The heat and mass transfer, pressure drop characteristics of stratified bubble flow for R134a/R245fa zeotropic mixtures in a rectangular channel with multi-cone staggered spoiler porous structure were studied experimentally. The bottom of channel was sintered with multi-cone staggered porous layer with thickness/height/diameter of 1/3/2 mm, respectively. The experiments were carried out under an inlet evaporation temperature of 20 ℃, and the ranges for heat and mass fluxes were 10–150 kW/m2 and 100–150 kg/(m2·s), individually. Compared to the rectangular channel with smooth porous structure, liquid mass transfer resistance of zeotropic mixtures in the rectangular channel with multi-cone staggered porous structure was decreased obviously. The decrease degree for Mixture 2 was largest, while the maximum average decrease was 89.61 % at G = 100 kg/(m2·s). The heat transfer coefficients were obviously improved and heat transfer enhancement increased gradually especially under conditions of high heat flux. The enhancement degree for Mixture 2 was the largest, while the maximum average increase was 26.05 % at G = 100 kg/(m2·s). The pressure drop increased significantly, and the increasing degree for Mixture 2 was smallest, while the minimum average increase was 38.18 % at G = 150 kg/(m2·s). Multi-cone staggered structure was helpful to reduce liquid mass transfer resistance and improve heat transfer performance for zeotropic mixture. The zeotropic mixture with higher liquid mass transfer resistance would have better heat transfer intensify and smaller increase of pressure drop with the help of multi-cone staggered structure.

Experimental study on flow boiling heat transfer performance of nanowire-printed substrates with porous-like structures

In this study, ZnO nanowires (NWs) were demonstrated to have considerable potential for heat transfer enhancement in flow boiling. Unlike the conventional NW fabrication method using metal-assisted chemical etching, we use the microcontact printing (µCP) method comprising solution-based growing methodologies to create ZnO NWs on different substrates, including bare Si-, Cu-, and Al-coated surfaces. This ZnO NW fabrication approach successfully overcomes the substrate limitation of typical NWs, such as Si- and Cu-NWs, which can be only formed on its intrinsic material following the high-cost fabrication approaches required by the metal catalyst. Additionally, a high pore-connected structure was observed on the interfacial area of the ZnO NWs, with the nanocore distribution area considerably enhanced by lengthening the NW length, beneficial to bubble nucleation and surface coolant supply during nucleate boiling. The length of the ZnO NW substrates is controllable by varying the growing period during the hydrothermal growth procedure. Further, the ZnO NW substrates reveal a remarkable enhancement in the flow boiling heat transfer regardless of the substrate variation, thereby exhibiting high variability in the substrate selection and potential of being used as the cooling approach for the thermal management of the high heat flux applications. We conducted a series of experiments related to the surface wettability, wicking, and bubble characteristics for different test substrates under saturated flow boiling conditions to explicitly determine the underlying mechanism in the heat transfer enhancement of the ZnO NWs.

Numerical Study of the Thermo-Hydro-Mechanical Coupling Impacts of Shallow Geothermal Borehole Groups in Fractured Rock Mass on Geological Environment

Fractured rock mass is extensively distributed in Karst topography regions, and its geological environment is different from that of the quaternary strata. In this study, the influences on geological environment induced by the construction and operation of a large-scale borehole group of ground source heat pumps are analyzed by a thermo-hydro-mechanical (THM) coupling numerical model. It was found that groundwater is redirected as the boreholes can function as channels to the surface, and the flow velocity in the upstream of borehole group is higher than those downstream. This change in groundwater flow enhances heat transfer in the upstream boreholes but may disturb the original groundwater system and impact the local geological environment. Heat accumulation is more likely to occur downstream. The geo-stress concentration appears in the borehole area, mainly due to exaction and increasing with the depth. On the fracture plane, tensile stress and maximum shear stress simultaneously occur on the upstream of boreholes, inducing the possibility of fracturing or the expansion of existing fractures. There is a slight uplift displacement on the surface after the construction of boreholes. The correlations of the above THM phenomena are discussed and analyzed. From the modeling results, it is suggested that the consolidation of backfills can minimize the environmental disturbances in terms of groundwater redirection, thermal accumulation, occurrence of tensile stress, and possible fracturing. This study provides support for the assessment of impacts on geological environments resulting from shallow geothermal development and layout optimization of ground heat exchangers in engineering practices.

Relationship between heat flux and bubble population density in heat transfer enhancement of flow boiling using microstructured surfaces

Relationship between heat flux and bubble population density in heat transfer enhancement of flow boiling using microstructured surfaces

Numerical Study of the Influence of Different Bending Shapes on the Heat Transfer Characteristics of Annular Cross Wavy Primary Surface Recuperator (CW-PSR)

The cross-wave primary surface recuperator (CW-PSR) is a dependable option as a recuperator for micro gas turbines (MGT). The micro CW-PSR studied in this paper is composed of 171 stacked curved plates, with each plate containing 33 micro heat transfer channels with equivalent diameters of less than 1 mm. In this study, the influence of bending curvature on the thermal performance of CW-PSR plates is investigated through three-dimensional numerical simulation with fluid–solid–thermal coupling. The results indicate that the variation in bending curvature studied can result in a noteworthy 8% difference in the total heat transfer coefficient of CW-PSR plates. A direct correlation between heat transfer capacity and secondary flow strength is derived mathematically, explaining the mechanism by which secondary flow enhances heat transfer. By employing this relationship, a comprehensive analysis of CW-PSR plates with diverse bending curvatures is conducted, effectively showcasing how curvature influences the secondary flow pattern and enhances the channel’s heat transfer capacity. In addition, this paper considers the comprehensive influence of the size parameters of the heat transfer unit and the bending curvature of the heat transfer plate on the heat transfer and flow characteristics of the CW-PSR, and a dominant mathematical expression is obtained, which can be used for the design of similar heat exchangers of the same type.

Parametric study of DBD plasma actuator for heat transfer enhancement in flow over a flat plate at low Reynolds numbers

The present study intends to find the optimal configurations of Dielectric-Barrier-Discharge (DBD) plasma actuators to augment the thermo-hydraulic performance in air flow over a flat plate at low Reynolds numbers. A range of parametric studies are conducted to determine the impact of the major contributing variables; Reynolds number, voltage frequency, and the applied voltage of the DBD actuator, on the heat transfer. This study reports that in the presence of plasma actuation, the factor of enhancement (Num/Num0) achieves about 3.76. Results show that reducing the Reynolds number and raising the frequency and applied voltage of the DBD actuator improve the factor of enhancement. Furthermore, the plasma system thermal efficiency (PSTE) to achieve the optimized electric power consumption and heat transfer augmentation is introduced as the rate of increase of enhancement factor to the rate of increase of consumed electric power. The frequency and applied voltage for the DBD actuator are used as the design variables to find the optimum configurations. Optimum electric power consumption and configurations of DBD plasma actuator based on Reynolds number are determined. Results also show that Vapp = 10.5 kV and f = 5 kHz conditions for DBD plasma actuator is much beneficial than the other configurations in the presence of low Reynolds number external flow. This study's findings will benefit designers in developing high-performance heat transfer.

A novel convective heat transfer enhancement method based on precise control of Gyroid-type TPMS lattice structure

Innovative single-phase flow heat transfer enhancement technologies can improve heat dissipation efficiency and reduce the weight and volume of heat dissipation equipment. Triply Periodic Minimal Surfaces (TPMS) hold promise as a outstanding single-phase heat transfer enhancement structure. To further improve the convective heat transfer performance of TPMS, this study independently presents a novel lattice structure control method specifically designed for Gyroid-type TPMS, along with the corresponding mathematical equation. By adjusting the parameter “α” in the equation, the Gyroid lattice geometry can be controlled. This study named “α” as the “Lattice deformation control parameter of Gyroid”. Numerical simulations indicate that increasing α from 0 to 0.25 within the Reynolds number range of 282–2579 can lead to a 19.9–28.9% increase in the average convective heat transfer coefficient (h¯) of the Gyroid, a 0.3%-4.5% increase in the Nusselt number (Nu), and a 13.0%-20.3% decrease in the average friction factor (f¯). The research results suggest that optimizing α is a promising new technology for enhancing convective heat transfer of Gyroid-type TPMS, as it not only improves its heat transfer performance but also reduces its flow resistance. The main reason for the convective heat transfer enhancement of Gyroid-type TPMS with increasing α is the increase in the heat transfer area, while the reduction of flow resistance is mainly due to the formation of the “flow-guiding structure” inside the Gyroid.

Two-phase secondary flow characteristics and heat transfer mechanism during boiling in a vertical helically coiled tube

A VOF model was used to study the two-phase secondary flow characteristics and the heat transfer mechanism in a helically coiled tube. According to the vapor-phase volume fraction, the helically coiled tube was divided into four sections for analysis, considering the different heat transfer characteristics in various boiling states. The heat transfer coefficient, velocity vector, and circumferential temperature distribution of the tube cross-sections were studied during boiling. The numerical results reveal that the secondary flow plays different roles in heat transfer of the four boiling states. When water boils in a helically coiled tube, the secondary flow enhances heat transfer in the subcooled and convective boiling sections, and alleviates heat transfer deterioration in the dry-out section. However, due to the secondary flow, the heat transfer coefficient drops rapidly by about 36% in the nucleate boiling section.

Effect of Utilizing Staggered Semi-Porous Fins on Channel Flow Heat Transfer Enhancement

A comprehensive numerical investigation of fluid stream and laminar forced convection were conducted to simulate the effects of the semi-porous fins, which are a new combination of porous and solid parts, inside a partially heated channel with a staggered arrangement. The present numerical approach is based on the Navier-Stocks and Darcy-Brinkman-Forchheimer equations in the fluid and semi-porous regions, respectively. The effects of various parameters such as Reynolds number, Darcy number, fin height, and aspect ratio of the porous region’s height to total fin height were studied. Also, the effects of these parameters are illustrated further as a variation of dimensionless pressure penalty and heat transfer enhancement ratio in the flow domain. The results demonstrate that using a new type of fin with both solid and porous parts in a single fin, enhances the heat transfer up to 6% compared to the porous fin and decreases the pressure penalty up to 23% compared to the solid fin at Darcy number of 10−4 when the percentage of aspect ratio is equal to 20%. Furthermore, it was found that the Darcy number of 10−5 is a critical value for the optimum utilization of semi-porous fins.

Influence of foamed metal core flow heat transfer enhancement on the performance of thermoelectric generators with different power generation characteristics

• Core flow heat transfer enhancement with foamed metal has been studied by experiment. • The effect of different PPI of the foamed metal has been analyzed. • The effect of different filling rate of the foamed metal has been analyzed. • PEC and OP are used to analyze the performance of enhancing heat transfer at core flow. With the development of science and technology, how to recycle the waste heat of high-temperature exhaust gas discharged from daily life and industry has become the focus of attention of the scientific community. Using thermoelectric generation technology to recover and utilize the waste heat of high-temperature gas of this part can effectively improve the efficiency of energy utilization. The thermoelectric generator (TEG) is modified by inserting the foamed metal copper (FMC) at the core flow field of the heat exchanger tube to enhance the heat transfer, and the heat exchange capacity of the heat exchanger can be effectively improved, so that the power generation efficiency of the TEG can also be effectively improved. At the same time, the core flow filling method can also reduce the increased amplitude of the back pressure of the TEG. Therefore, in this article, experiments have been conducted to investigate the effect of improved heat transfer of FMC at the core flow field on the performance of TEG with different power generation characteristics. We first evaluate the effect of different enhanced heat transfer conditions on the heat transfer capacity of TEG hot end heat exchanger with the Performance Evaluation Comprehensive (PEC) evaluation method. The results showed that the enhanced heat transfer by the FMC at the core flow field can significantly improve the heat transfer capacity of the heat exchanger. Then we use the Overall Performance (OP) evaluation method to evaluate the effect of TEG on its performance under different enhanced heat transfer conditions. The results showed that the power generation of the TEG filled with FMC at the core flow field is higher than that of the unfilled TEG, and the filling of FMC with a high filling rate and high pore density can significantly improve the power output and power generation efficiency of the TEG. However, by evaluating the TEG filled with FMC at the core flow field through the OP evaluation method, it was found that the use of FMC with a low filling rate and low pore density can better improve the overall performance of TEG.

Evaluation of Thermal Hydraulic Characteristics of a Two Phase Superhydrophobic Microfluidic Device using Volume of Fluid Method

ABSTRACT The slug flow in a superhydrophobic microchannel with a T-junction was studied computationally. The continuous phase passed through the main channel while the dispersed phase 1 1. was introduced through the side channel. The volume of fluid (VOF) was employed to track the interface to study the dynamics of slug flow. First, a mesh independence study was carried out to select the optimum mesh by comparison of CFD results with experimental data. The developed model of microchannel was used to study slug flow heat transfer enhancement for micro cooling of electronic chips. The constant heat flux was applied on the walls of the microchannels and the axial wall temperature profile was noted. Upon quantification of heat transfer augmentation in terms of wall temperature reduction, Nusselt number and heat transfer coefficient enhancement, it was noted that slug flow performed much better vis-à-vis single-phase flows at similar conditions.

Experimental study on the effect of core flow heat transfer enhancement on the performance of TEG

Using thermoelectric power generation technology to recycle and utilize the waste heat of high temperature gas can effectively improve the efficiency of the energy. Filling foam metal (FM) at the core flow field of the thermoelectric generator (TEG) can effectively enhance the heat transfer of the TEG passage hot end and reduce the pressure loss, which can further optimize the power generation efficiency of TEG. Therefore, this article compares the effect of thermoelectric module with different performance on the power generation efficiency of TEG under the enhanced heat transfer mode of core flow filled with FM through experiments. The results showed that when the TEG passage hot end partially filled with FM at core flow field, the power output and power generation efficiency of the TEG can be significantly increased. And when high figure of merit (ZT) thermoelectric module is used in TEG, the overall performance of the TEG is also greatly improved.

Effect of core flow heat transfer enhancement on power generation characteristics of thermoelectric generators with different performances

In this study, the effect of enhancing the core flow heat transfer with metal foam on the performance of thermoelectric generators with different power generation characteristics is studied experimentally. Filling the core flow area of the gas channel in a thermoelectric generator with metal foam can greatly improve the heat transfer capacity of the gas channel with a small pressure loss, thereby improving the power generation efficiency. The results show that, first, the heat transfer enhancement achieved by partially filling the core area of the gas channel with metal foam can significantly improve the performance of thermoelectric generators, the maximum output power is about 1.5 times higher than that of the unfilled channel. Second, for a thermoelectric generator with different modules, the friction coefficient for different filling ratios increases by about 16 times at most, while the Nu value increases by only three times at most, and according to the PEC of the gas channel, metal foam with high filling rate and low pore density is more suitable for the thermoelectric generator. Third, it is more appropriate to use the thermoelectric module with a high figure of merit as the selection criterion for deciding whether to adopt the technique of enhancing heat exchange through the gas channel. The maximum output power and efficiency of the thermoelectric generator using the high figure of merit module are 300% and 160% higher than those of the thermoelectric generator using the low figure of merit module, respectively.

A comprehensive computational framework for repurposing FDA-approved drugs to interfere with Pseudomonas aeruginosa quorum sensing

The effect of pulsatile flow on the momentum and heat transfer characteristics for a sphere in power-law fluids is investigated numerically otherwise in the steady regime. Extensive results are presented in terms of streamlines, isotherms, pressure coefficient and local Nusselt number, drag coefficient and Nusselt number over the range of conditions as: Reynolds number (5 ≤ Re ≤ 120), Prandtl number (0.7 ≤ Pr ≤ 100), power-law index (0.3 ≤ n ≤ 1.5), frequency (π/4 ≤ ω* ≤ π) and amplitude of pulsations (0 ≤ A ≤ 0.8). The vortex grows during the deceleration phase of the pulsatile cycle and vice-versa. The pulsatile flow enhances heat transfer in shear-thinning fluids by 30% (at Re = 120, Pr = 100) over that of uniform flow conditions whereas the corresponding improvement is smaller than this value for shear-thickening fluids, i.e., n > 1. This is accompanied by a marginal increase in drag.

Pulsatile flow of power-law fluids over a sphere: Momentum and heat transfer characteristics

The effect of pulsatile flow on the momentum and heat transfer characteristics for a sphere in power-law fluids is investigated numerically otherwise in the steady regime. Extensive results are presented in terms of streamlines, isotherms, pressure coefficient and local Nusselt number, drag coefficient and Nusselt number over the range of conditions as: Reynolds number (5 ≤ Re ≤ 120), Prandtl number (0.7 ≤ Pr ≤ 100), power-law index (0.3 ≤ n ≤ 1.5), frequency (π/4 ≤ ω* ≤ π) and amplitude of pulsations (0 ≤ A ≤ 0.8). The vortex grows during the deceleration phase of the pulsatile cycle and vice-versa. The pulsatile flow enhances heat transfer in shear-thinning fluids by 30% (at Re = 120, Pr = 100) over that of uniform flow conditions whereas the corresponding improvement is smaller than this value for shear-thickening fluids, i.e., n > 1. This is accompanied by a marginal increase in drag.

Laminar flow heat transfer enhancement in square and rectangular channels having: (1) A wire-coil, axial and spiral corrugation combined with helical screw-tape with and without oblique teeth and a (2) spiral corrugation combined with twisted tapes with oblique teeth

Heat transfer enhancement is a vital area of research as it helps to design heat exchangers of reduced sizes and improved heat transfer rates. Thus, there is a pressing need for techniques which result in significantly increased heat transfer over and above increased pressure drop. In this paper, the benefits of different compound techniques using (1) wire-coil inserts and helical screw-tape inserts with oblique teeth, (2) wire-coil inserts and helical screw-tape inserts without oblique teeth, (3) spiral corrugation and helical screw-tape inserts without oblique teeth, (4) axial corrugation and helical screw-tape inserts without oblique teeth, and (5) spiral corrugation and twisted-tape inserts with oblique teeth were studied experimentally for laminar flow in a non-circular duct (aspect ratio = 1, 0.5 and 0.33). Servotherm medium oil with Prandtl number ranging from 430 to 530 was used as working fluid. The experiment was conducted for a uniform heat flux boundary condition. The effect of geometric parameters of the inserts such as wire-coil diameter, wire-coil helix angle, tooth angle and tooth horizontal length of the screw tape and the twisted tape, corrugation height and corrugation pitch, etc. on heat transfer and pressure drop characteristics was presented. The influence of duct geometry (aspect ratio) on Nusselt number and friction factor was also studied. Correlations for Nusselt number and friction factor were developed and a performance evaluation was done.

Experimental research of heat transfer in supersonic separated compressible gas flow

The results of an experimental study of heat transfer for supersonic flow around plane surface in the wake of a rib are presented. The study was conducted on unsteady regime during the launching supersonic wind tunnel before reaching the equilibrium thermal state. The initial flow Mach number was 2.2, Reynolds number based on the length of the dynamic boundary layer from the nozzle throat was over 20 million at the nozzle exit section. The rib height was varied from 2 to 10 mm while boundary layer thickness for smooth model flow in the region of rib placement was about 6 mm. Recovery temperature and the coefficient of heat transfer enhancement for flow past the rib are presented in comparison with the regime of a smooth model flow. The research was carried out with the use of thermocouples with thermal compensation, total and static pressure probes, LabView automation programs.