Mechanism Of Heat Transfer Enhancement Research Articles

Numerical simulation of nitrogen vapor condensation on microstructure surface

• A three-dimensional model of condensation on micro fin is established. • Micro fin surface area exhibits different functionalities. • Micro fin curvature radius affects the driving force of liquid film movement. • An optimal micro fin structure for best performance is proposed. The mechanism of the condensation heat transfer enhancement on the microstructure surface has not been deeply explored, which restricts the full play of microstructure strengthening ability. In this study, a three-dimensional numerical simulation of the condensation on the micro fin surface revealed the liquid film flow behavior and the heat transfer performance, and the model is in good agreement with the experimental and theoretical results. The internal flow field on the curved surface shows that the liquid film is redistributed, and it proves that the surface tension plays a leading role in promoting the transverse movement of the liquid film. The micro fin surface area exhibits different functionalities, the excellent mass transfer region is located at the fin top, and its local heat transfer coefficient can reach 25 k W · m - 2 · K - 1 , which is one order of magnitude higher than Nusselt's theoretical value. The fin bottom is the condensate collection area, where the condensate discharge velocity is increased by 2–3 times. The simulation results of the different micro fins indicate that the curvature radius of the micro fin could affect the condensate flow process, and the aspect ratio of the micro fin determines the local heat transfer enhancement degree and the strengthening area ratio. This study finally pointed out that the high-efficiency micro structure should have a large area of the fin top for enhancing heat transfer, as well as a large volume of the fin bottom for the liquid drainage.

Flow boiling heat transfer characteristics on micro-pin-finned surfaces in a horizontal narrow microchannel

In this paper, an experimental investigation of flow boiling in a horizontal narrow microchannel with hydraulic diameter of Dh = 952 μm was conducted. Flow boiling heat transfer characteristics of micro-pin-finned surfaces were evaluated over the mass fluxes range from 200 - 500 kg/m2s (corresponding flow velocity: 0.2 - 0.5 m/s), using deionized water as the working fluid with different inlet temperatures Tin = 30 - 50 °C. The outlet pressure of the microchannel was about 1 atm. The boiling curves, heat transfer coefficients, bubble behaviors and heat transfer enhancement mechanism were discussed with the variation of heat flux, mass flux and inlet temperature. The results indicated that the micro-pin-finned surfaces could improve the critical heat flux (CHF) and heat transfer coefficient (HTC) greatly due to numerous nucleate sites and large heat transfer area enhancement ratio. The CHF up to 360 W/cm2 has been demonstrated at a mass flux of 500 kg/m2s. Compared with a smooth surface, a 240% higher heat transfer coefficient (18.4 W/cm2K) on the S30-120 surface has been achieved at a mass flux of 500 kg/m2s with an inlet temperature of 30 °C. The boiling curves of the micro-pin-finned surfaces presented an obvious “hook back” phenomenon after reaching the onset of the nucleate boiling (ONB) and then the wall temperature had a slight rise with the increase of the heat flux. Moreover, the intensive micro-pin-finned arrangements showed a significant wicking effect and promoted liquid replenishment, which could destroy the liquid boundary and enhance turbulence. The bubble behaviors were also captured to explain the heat transfer enhancement mechanism in a horizontal narrow microchannel.

Experimental and numerical studies on heat transfer enhancement in the silicothermic reduction process

Currently, the silicon thermal method is the main process to produce primary magnesium, which is controlled by chemical reaction kinetics and the heat transfer process in the retort. However, the unbelievably low thermal conductivity and point contact thermal resistance of the pellets greatly reduce the heat transfer efficiency, resulting in low magnesium production efficiency. To increase productivity, the enhanced heat transfer mechanism was studied in detail by experiments in this paper. The results show that temperature rise may not accurately reflect the enhanced heat transfer effect, thus a synthetical valuation method which took total heat transfer, effective heat transfer ratio and temperature rise into account is proposed. On this basis, a new recycling technique which uses high temperature silicon carbide particles to enhance the heat transfer in the retort is developed. Then a three-dimensional unsteady numerical model incorporating the chemical reaction kinetics and heat transfer was established and used to prove the superiority of this new technique in terms of heat transfer efficiency, magnesium production rate and production cycle. Numerical results show that when the initial temperature of silicon carbide is 1373 K, the instantaneous magnesium production rate can reach 13.2 kg/h within 30 min of heating, which is three times that of the traditional silicothermic process. The production cycle could also be shortened to 240 min, which is half of the original, and the production efficiency can be doubled. This new technique can significantly improve heat transfer efficiency, shorten the production cycle and reduce the production cost.

Thermodynamic evaluation and particle migration of hybrid nanofluids flowing through a complex microchannel with porous fins

A compound method is established by adding Al2O3-TiO2/Water (W) nanofluids and porous media in a microchannel to reduce pressure drop and enhance heat transfer in the limited heat dissipated area. Effects of porosity and hybrid nanofluids are numerically investigated in the laminar region. Pressure drop, Nusselt number, velocity and temperature fields are used as indicators to evaluate the characteristics of flow and heat transfer. Furthermore, the analytical methods including thermodynamic evaluation and nanoparticle migration were performed to reveal the essential mechanism of heat transfer enhancement. Compared with non-porous microchannels, the maximum reduction of porous microchannels is 9.73% (ε = 0.5), 8.3% (ε = 0.6) and 6.8% (ε = 0.7) for pressure drop, while the maximum agument is 23.53% (ε = 0.5), 20.59% (ε = 0.6) and 16.67% (ε = 0.7) for Nusselt number. The permeability of fluid reduces pressure drop and enhances heat transfer in the porous region. The maximum heat transfer irreversibility reduces up to 64%, and exergy efficiency reaches 88% by adding nanofluids and porous media. Nanoparticle migration originating from thermophoresis affects the concentration distribution near the wall, and cold-hot fluids fully mix in the porous region, thereby increasing heat transfer. The effect of thermophoretic diffusion of dilute nanofluids in micro-scale convective heat transfer is higher than that in the macro-scale one.

Influence of key geometry parameters on heat transfer and flow resistance properties of toothed internal spiral piece cracking furnace tube

Strengthening the heat transfer technology of the furnace tube in the radiant section of the cracking furnace is very important in the field of energy-saving and environmental protection. This research proposes a new type of open-tooth spiral to enhance heat transfer and increase ethylene productivity. The mathematical computation has been conducted to evaluate the thermal-hydraulic performance with an open-tooth spiral. The classical Ordinary tube was also investigated for comparison. At the same time, the vortex identification method based on the Q criterion reveals the enhanced heat transfer mechanism of the open-toothed spiral tube. Then use the field synergy principle to qualitatively judge the heat transfer performance, combined with the comprehensive heat transfer evaluation factor (PEC) to quantitatively evaluate the comprehensive heat transfer performance of the open-tooth spiral tube. The results show that although the Nusselt number of the open-tooth spiral is 8%-10% lower than that of the ordinary screw, the friction resistance coefficient of the spiral tube with an opening angle of 15° can be reduced by 33%∼43%. Its PEC value is between 1.15 and 1.25, with excellent comprehensive heat transfer performance. It can effectively improve key products, such as ethylene, propylene, etc.

Vibration-Enhanced Heat Transfer of Helical Tube with Different Number of Tubes

Based on a two-way fluid/solid coupling model, the heat transfer enhancement mechanism of a helical elastic coiled tube (HECT) by fluid-induced vibration (FIV) was numerically studied. The field synergy principle was used to compare and analyze the shell-side convective heat transfer of the HECT. The field synergy angle, Nusselt number, and comprehensive enhanced heat transfer performance were quantitatively studied from the overall and local perspectives. The influences of the Reynolds number and the number of tubes on heat transfer performance and vibration-enhanced heat transfer performance were analyzed. Numerical results show that the field synergy level and the oscillation relative velocity near the tube are two important factors for fluid-induced tube vibration to enhance heat transfer. The average Nusselt number of coiled tubes decreases when the number of tubes increases and increases when the Reynolds number increases. The comprehensive heat transfer enhancement capability of the heat exchanger with different numbers of tubes of 2, 4, and 8 gradually decreases as the number of tubes increases, and the average performance evaluation criteria of the heat exchanger are 1.043, 1.023, and 1.004, respectively, for the three typical cases. The tubes on the upper and lower sides have a better heat transfer enhancement effect by the FIV.

Performance of a Double‐Tube Heat Exchanger with Staggered Helical Fins

AbstractA novel double‐tube heat exchanger with staggered helical fins (DTHE‐SHF) is proposed. The mechanism of drag reduction and heat transfer enhancement was analyzed based on the theory of field synergy. Numerical simulations were conducted to study the thermal performance of the DTHE‐SHF. The effects of torsion angle and number of the SHF were investigated. The results indicated that, compared with the double tube heat exchanger with traditional straight helical fins (DTHE‐TSHF), the pressure drop of the DTHE‐SHF is reduced significantly, and the comprehensive performance of the DTHE‐SHF is improved by 10 %–30 %. The average synergy angle between the velocity field and the pressure field is optimized.

Experiments on Single-Phase Nanofluid Heat Transfer Mechanisms in Microchannel Heat Sinks: A Review

For more than 20 years, the use of nanofluids to enhance heat transfer in microchannel heat sinks (MCHSs) has been the subject of a large number of scientific articles. Despite the great potentialities reported in several works, the presence of controversial results and the lack of understanding of heat transfer enhancement mechanisms prevent further advancement in the use of nanofluids as coolants. This article reviews the scientific literature focused on several aspects of nanofluids that have a role in the heat transfer enhancement within the MCHSs: nanofluid stability, thermal conductivity, and particle clustering, as well as the particle–surface interactions, i.e., abrasion, erosion, and corrosion. We also include the most relevant works on the convective heat transfer and MCHSs operated with nanofluids in our review.

Enhanced heat transfer mechanism and flow topology of a channel contained with semi-circular hinged V-shaped baffles

The current work presents a numerical analysis of heat transfer mechanism and flow topology of channels with V-shaped baffles (VB) and semi-circular hinged V-shaped baffles (SH-VB). The vital purpose of applying SH-VB roughened channels is to induce combined vortex and impinging jets flows and reduce the pressure drop in the channel as compared to the use of VB. The effects of geometric parameters: pitch ratios (PR = 0.5, 1.0, 1.5 and 2.0), blockage ratios (BR = 0.1, 0.15 and 0.2), semi-circular hinge angles (β = 5°, 10° and 15°) were studied in the turbulent region (3000 ≤ Re ≤ 19,000). The study was performed at constant aspect ratio (AR) of 10.0 and V-baffle attack angle of α = 45°. The numerical results showed that the decrease of PR and increase of BR resulted in an increase of heat transfer enhancement and pressure drop. The maximum heat transfer enhancement of 5.57 times over the smooth rectangular channel was found at PR = 0.5, BR = 0.2, β = 10°, and Re = 3000. The numerical results also indicated that SH-VB gave higher heat transfer and thermal enhancement factors (TEF) and lower pressure losses than VB. SH-VB provided higher Nu/Nu0, lower f/f0 and higher TEF than VB by around 1.73–8.98%, 5.41–26.84% and 4.51–15.22%, respectively depending on Re, BR, PR and β. The optimum parameters of SH-VB which gave the maximum TEF at 2.6, were BR = 0.10, PR = 0.5 and β = 10° at Re = 3000.

Heat transfer and flow structure in a plane diverging channel

Heat transfer in diverging channels is essentially different from the one in channels of constant cross section. The mechanism of heat transfer enhancement in diverging channels compared to the channels of constant cross section is based on increased turbulence intensity. The present paper suggests new approach to the analysis and generalization of heat transfer data. Heat transfer on the wall of a diverging channel should be considered similarly to heat transfer from a plate instead of heat transfer in a channel. The data on heat transfer coefficient on the wall of a plane diverging channel at different expansion angles can be generalized by the Stanton number as a function of the Reynolds number and Kays acceleration parameter, St∼f(Re, K). This function has characteristic sections along the wall that are typical of laminar, transitional and turbulent boundary layers. Stanton and Reynolds numbers based on the distance from the inlet and the velocity at the diverging channel inlet can be employed for data generalization in all the considered ranges of geometries and flow regimes. Kinematic structure of flow is examined to reveal hydrodynamic mechanisms behind heat transfer formation. Optical measurements provide the profiles of velocities and turbulence parameters in characteristic sections of the channel. Mechanism of turbulent structure formation in diverging channels is examined. Correlation between hydrodynamics and heat transfer is analyzed. Correlation between local coefficients of heat transfer and near-wall Reynolds stress is revealed.

A Numerical Study on Heat Transfer Enhancement and Fluid Flow of Enhanced Tube With Ellipsoidal Dimples and Protrusions

Abstract Heat transfer enhancement is an important thermal–hydraulic phenomenon in heat exchangers. Previous investigations mostly focused on unidirectional heat transfer enhancement inside the tube. In order to realize the bidirectional heat transfer enhancement inside and outside the tube, an enhanced tube with ellipsoidal dimples and protrusions is proposed. Through comparison among three computational conditions, the non-conjugate and periodic condition is adopted in this paper. Flow characteristics and heat transfer performance of the enhanced tube are investigated by numerical method. The distributions of temperature and flow fields are carried out to illustrate the heat transfer enhancement mechanisms. The Nusselt number, friction coefficient, and performance evaluation criteria (PEC) are presented to reveal thermal–hydraulic performance. Moreover, the effects of the geometric parameters such as dimple depth, dimple pitch, dimples/protrusions numbers in the circumference, and arrangement mode on thermal–hydraulic characteristics are investigated. The results show that the structure of dimples and protrusions induces intensive impingement and interrupts the boundary layers, improving the heat transfer for the internal and external flow, respectively. Besides, the Nu increases with depth and dimples/protrusions numbers in the circumference and decreases with the pitch in most cases. The overall heat transfer performance of staggered arrangement is better than that of aligned arrangement. For the present parameter conditions, when D = 2 mm, P = 25 mm, n = 6, and Re = 5000, the largest PEC of 1.40 and 1.42 occurs for the internal and external flow, respectively.

Impacts of two-phase nanofluid approach toward forced convection heat transfer within a 3D wavy horizontal channel

Combined approach of geometrical variations to increase heat transfer surface with the application of high thermal conductive nanofluids can influence heat transfer enhancement in new thermal systems. This study numerically investigates the thermophoresis and Brownian diffusion as convective heat transfer enhancement mechanisms with two-phase Al2O3-water nanofluid in a 3D wavy horizontal channel having upper and lower wavy surfaces imposed with a uniform temperature and adiabatic conditions, respectively. Empirical correlations are used to model the thermal conductivity and viscosity as the most influential thermophysical properties of nanofluid. A commercial CFD code based on the Finite element Method (FEM) is employed to solve the governing equations in curvilinear coordinates for the computational domain. The employed dimensionless parameters include Reynolds number (Re), nanoparticle concentration (ϕ), channel’s number of oscillations (N) and amplitude (A). The current results correlate reasonably with similar experimental results in the literature. The results show that nanofluid flow in the wavy-channel augmented heat transfer, especially at increasing nanoparticle volume fraction and Reynolds number. Also, higher convective heat transfer is attained with increasing flow mixing due to increase in oscillation frequency. Local Nusselt number of nanofluid significantly rises when the Re varied from 200 to 1000 by about 75% at ϕ=0.02, N=4 and A=0.1.

Investigation of fluid flow and heat transfer characteristics in a microchannel heat sink with double-layered staggered cavities

This paper is focused on a numerical investigation of fluid flow and heat transfer characteristics in a microchannel heat sink (MCHS) with double-layered staggered cavities (MCHS-DLSC). The effects of both the expansion-to-contraction ratio (φ) and the height ratio (β) on the thermal performance are studied by numerical simulation. A comparison is performed between the MCHS-DLSC and the MCHS with straight channels (MCHS-SC). The comprehensive performance of the MCHS-DLSC is evaluated. The flow, temperature, and pressure fields are employed to discern the mechanism of heat transfer enhancement for the MCHS-DLSC. The results indicate that the Nusselt number (Nu) and friction factor (f) of the MCHS-DLSC are 1.03–1.85 and 1.37–1.41 times higher than those of the MCHS-SC, respectively. However, the thermal resistances of the MCHS-DLSCs are at most 25.9–33.8% less than that of the MCHS-SC. The Nu, f, and thermal enhancement factor (PEC) decrease with the increase of the φ, while the R increases as the φ increases. However, when the β = 0.5, the Nu, f, and PEC reach maximum values at fixed φ under the corresponding Reynolds number, but the R is the minimum value simultaneously. It is confirmed that the DLSC can strengthen the fluid mixing, interrupting, and redeveloping of thermal boundary layers under the action of the jet and throttling effects. Hence, the DLSC has effective heat transfer enhancement.

A Thermohydraulic Performance of Internal Spiral Finned Tube Based on the Inner Tube Secondary Flow

In this article, the BSL k-ω model was chosen as the turbulence model to simulate the heat transfer and flow characteristics of the proposed tubes inserted with internal spiral fins when the Re was set as 3000 to 17,000. The numerical results agreed well with the empirical formula. The average deviations of Nu and f between the simulation results and empirical formula results were 5.11% and 8.45%, respectively. By means of numerical simulation, the impact of three configurational parameters on the thermal performance was studied, namely the pitch P, the height H, and the number N of the internal spiral fins. The results showed that the Nu and f of the internal spiral finned tube were 1.77–3.74 and 3.04–10.62 times higher than those of smooth tube, respectively. PEC was also taken into account, ranging from 1.038 to 1.652. When the Re was set as 3000, the PEC achieved the peak value of 1.652 under the height H of the fins at 5 mm, the number N was 8, and the pitch P was 75 mm. However, with the increase of Re, the effect of pressure drop on the comprehensive performance in the tube was stronger than that of thermal enhancement. However, the PEC gradually decreased as the Re increased from 3000 to 17,000. In addition, the velocity and temperature fields were obtained to investigate the mechanisms of heat transfer enhancement.

Simultaneous Measurement of Flow and Thermal Fluctuations near Reattachment Point over a Backward Facing Step

In this study, the mechanism of heat transfer enhancement near the reattachment point behind a backward facing step is experimentally investigated at Reynolds number of 5400. The unsteady fluctuations of the velocity and temperature are simultaneously measured by using 2 dimensions and 3 components (2D-3C) Particle Image Velocimetry (PIV) and the high-speed infrared (IR) thermography. In order to discuss the temporal-spatio characteristic of the velocity and the heat transfer fluctuations, 3 components of the velocity and the heat transfer coefficient are calculated. Also, these time history data are analyzed by using continuous wavelet transform and the fluidic and thermal structures are visualized. As a result, the characteristic frequency of the velocity and heat transfer coefficient appears at the non-dimensional frequency of 0.06 and 0.2. These frequencies have relationship to the fluctuations of the streamwise vortex and the reverse region.

Study of Unsteady Thermal and Flow Structure near Reattachment Point behind a Backward Facing Step with POD Analysis

In this study, the mechanism of heat transfer enhancement near the reattachment point behind a backward facing step is experimentally investigated at a Reynolds number of 5400. The unsteady fluctuations of the velocity and temperature were simultaneously measured by using 2 dimensions and 3 components (2D-3C) Particle Image Velocimetry (PIV) at x/H = 4.5 and the high-speed infrared (IR) thermograph on the heated wall. In order to discuss the thermal and flow structure near the reattachment point, 3 components of the velocity and the heat transfer coefficient were calculated. The spatial distribution of the flow and thermal fluctuations was reconstructed by using proper orthogonal decomposition (POD) analysis. As a result, the characteristic structure of the flow and thermal fluctuations appeared for the lower number of POD modes. These downwash and upwash flow in the transverse direction corresponded to the increasing and decreasing of the heat transfer coefficients at x/H of 4.5.

Mechanism of Heat Transfer Enhancement of Supercritical Pressure Co2 in a Horizontal Variable Cross-Section Circular Tube Under Cooling Conditions

Mechanism of Heat Transfer Enhancement of Supercritical Pressure Co2 in a Horizontal Variable Cross-Section Circular Tube Under Cooling Conditions

Mechanism of Heat Transfer Enhancement of Supercritical Pressure Co2 in a Horizontal Variable Cross-Section Circular Tube Under Cooling Conditions

Mechanism of Heat Transfer Enhancement of Supercritical Pressure Co2 in a Horizontal Variable Cross-Section Circular Tube Under Cooling Conditions

Mechanism of Heat Transfer Enhancement of Supercritical Pressure Co2 in a Horizontal Variable Cross-Section Circular Tube Under Cooling Conditions

Mechanism of Heat Transfer Enhancement of Supercritical Pressure Co2 in a Horizontal Variable Cross-Section Circular Tube Under Cooling Conditions

Swirl flow heat transfer and flow characteristics in a solid and permeable pipe with exit nozzle

This paper goal is to study swirling flow heat transfer and flow characteristics in the round solid and permeable pipe with an exit conical nozzle. The experiments were carried out in the pipe with an internal diameter of 80 mm, 1200 mm long, and an exit conical nozzle ratio of 0.5, 0.63, and 0.75. Seven swirl generators with profiled blades were tested at different flow conditions. The flow experiments in the solid pipe were carried out at Red = 42 000 –170 000, while heat transfer experiments - at Red = 30 000 – 150 000. In the permeable pipe the flow experiments were conducted at Red = 70 000 –130 000, heat transfer experiments – at Red = 50 000 – 120 000 and blowing ratio B* = 0.002 – 0.050. The heat transfer coefficients, total pressure losses, and swirl flow parameters . were presented. In the pipe with exit nozzle ratio over 0.75 the flow parameters are identical to those in the open pipe with flow swirl. At the lower exit ratio (0.5) and high swirl flow rate, the exit nozzle restricts the reverse flow from outside and radically changes the axial and rotational speed distribution inside the pipe. A few novel scientific results were obtained in the pipe with an exit nozzle, among them: the powerful vortex formation in the near-axis area of pipe, «partially suppressed» turbulent heat transfer pattern near the swirl generator, identical mechanism of heat transfer enhancement in the solid and permeable pipe, common relations linking the local and total swirl flow numbers, .axial and rotational speed. A few novel equations for heat transfer and flow parameters . were presented as a result of experimental data processing.