Heat Transfer Enhancement Methods Research Articles

Utilizing subsonic vibration in cryogenic quenching for heat flux enhancement

ABSTRACT An active method for heat transfer enhancement is proposed and thoroughly characterized for the first time for cryogenic quenching, in which mechanical vibrations are directly induced onto the sample to be quenched, employing a compact modular system composed of a pneumatic plunging device along with a DC motor/eccentric cam mechanism, generating oscillations on three different axes at a wide array of amplitude-frequency combinations. The quenching performance was investigated from both flow-field and thermal standpoints, through numerical simulations of the flow behavior, high-speed imaging of the process, and experimentally generated transient quenching and boiling curves under the influence of the governing parameters, underlining their combined effect on the mechanism of heat transfer augmentation. Under optimal conditions of amplitude and frequency, a remarkable CHF enhancement of 270% was obtained and the quenching time was reduced from 5 seconds to 1.8 seconds compared to the non-vibration case, where the vibration frequency was found to be the primary factor involved in the increase of oscillation-driven pressure drop and subsequent cavitation-induced CHF enhancement. Consequently, we are providing researchers in the field of cryogenics a method that not only provides enhanced performance but also offers flexibility enabling active control of the governing parameters of the system.

Heat transfer characteristics of ceramic foam/molten salt composite phase change material (CPCM) for medium-temperature thermal energy storage

Molten salts have been widely used as energy storage materials in medium- and high-temperature thermal energy storage. However, pure salt commonly suffers from low thermal conductivity and many conventional methods of heat transfer enhancement do not apply due to the serious corrosion and the extremely high temperature. In the current study, the open-cell SiC ceramic foam was integrated with solar salt (60wt% NaNO3 + 40wt% KNO3) to enhance the heat transfer of salt and avoid severe corrosion issues. A visualised experiment was for the first time conducted to investigate the melting phase change heat transfer in the ceramic foam/molten salt composite phase change material (CPCM). A representative elementary volume (REV)-scale simulation was simultaneously performed and the computational results were compared with experimental data. It is found that the conduction-friendly ceramic skeleton remarkably enhances the heat transfer in molten salt, especially in the region far away from the heat source. The spatial temperature difference across the composite is decreased in both horizontal and vertical directions and the local superheating is mitigated. The enhancement of heat conduction is greater than the suppression of natural convection; as a result, the melting rate of the CPCM is increased by 41.3%. This study provides crucial benchmark data of phase change heat transfer for medium-temperature thermal energy storage and paves the way for system design and optimization.

Heat transfer enhancement by pulsating flow of a viscoelastic fluid in a microchannel with a rib plate

ABSTRACT In order to improve the heat transfer performance of the microchannel heat exchanger, a composite heat transfer enhancement method was proposed. Viscoelastic fluid was used as working fluid in pulsating flow condition, and rib plates were added to the microchannel to bring extra disturbance to the flow. The Oldroyd-B constitutive model of the viscoelastic fluid was used in the numerical simulation, and the flow field, temperature field, Nusselt number (Nu), and pressure drop were analyzed when the average Reynolds number (Re) is 10. Both Strouhal number (St) and amplitude are important factors affecting heat transfer, but they have an insignificant influence on pressure drop at low Reynolds number. The St = 0.125 and amplitude A = 0.8 are better parameters. The increase of Weissenberg number (Wi) will cause the vortex to split into several subsidiary vortexes during its development, which will also develop to various positions in the channel, thus further enhancing the heat transfer. When the Wi is in the range of 1 ~ 5, the performance evaluation criteria rises at a relatively fast rate, from 1 to 1.404.

Heat transfer characteristics and compatibility of molten salt/ceramic porous composite phase change material

Molten salts are popular energy storage materials for medium- and high-temperature thermal energy storage. However, current methods for heat transfer enhancement do not apply due to the high corrosivity of molten salts and extreme high-temperature environment. Herein, porous silicon carbide (SiC) ceramic and solar salt are formed into a composite phase change material (PCM). The ceramic skeleton is fabricated to an open-cell structure with high porosity and a large pore configuration. The SiC ceramic is wetted by the solar salt, so that the impregnation of salt into ceramic achieves a high loading at atmospheric pressure. The result of visual inspection shows that the temperature distribution in the composite PCM is more uniform compared with pure solar salt. The maximum temperature difference is reduced from 148 ℃ to 130 ℃ and the overall phase change rate is increased by up to 42.9 %. The SiC ceramics show excellent corrosion resistance during the cycles of thermal charging and discharging as compared to copper and aluminium which are the widely used thermal promoters for low-temperature PCMs. The results obtained from reactive molecular dynamics (MD) simulation are consistent with the corrosion behaviours of SiC ceramic in solar salt from aspects of physical dissolution, chemical reaction, and thermal stress failure. The composite PCM has advantages of simple preparation, low cost, and easy maintenance, therefore it has a great potential for large-scale applications.

Experimental and numerical study of the thermal-frictional behavior of a horizontal heated tube equipped with a vibrating oscillator turbulator

This study investigates a novel heat transfer enhancement method named electromagnetic vibration (EMV) method. For the first time a stretched oscillator with geometries of string and strip was placed coaxially inside a heated copper tube and induced to vibrate at the natural frequencies. Longitudinal vibration of the stretched oscillators creates harsh turbulence, radial flow and vortexes inside the heated tube. Tests were conducted for various vibrational frequencies ranging 0 to 1 KHz in different mass flow rates. The experimental results indicated that oscillator with the geometry of strip has the better thermal performance; also the heat transfer coefficient can be increased with increasing vibration frequency from 2.18 to 2.50 folds. For comprehensive analyze at high frequency vibrations, a numerical model was conducted for the vibrating strip geometry to evaluate this new method capability in heat transfer performance enhancement. The numerical study shows that at 1 KHz frequency, heat transfer coefficient and thermal enhancement factor (TEF) can reach up to 32.2 and 14.2 fold compare to the plain tube, respectively. This magnificent increment shows the high potential of the EMV method as a new heat transfer augmentation technique.

Single-phase flow heat transfer characteristics in helically coiled tube heat exchangers

Abstract The aim of this review is to present a summary of the published papers of the heat transfer and pressure drop characteristics for single-phase flow in helically coiled tubes. The effect of geometrical parameters such as curvature ratio, coil pitch and working conditions such as Reynolds number, Dean number, flow rate and flow arrangement on heat transfer and pressure drop in helically coiled tubes are determined in the light of the experimental, numerical and analytical studies in the literature. Also, the effect of using nanofluids in comparison with conventional fluids, using enhanced surfaces such as corrugated, micro-finned, dimpled with regards to smooth surfaces and wire coil insert usage in helically coiled tubes are discussed. The correlations proposed for determination of Nusselt number and friction factor in helically coiled tubes are presented in detail separately under laminar and turbulent flow regimes. The studies show that usage of helically coiled tube merely gives higher heat transfer rate and pressure drop in comparison to straight one, additionally, the heat transfer performance increases with the inclusion of the combination of other passive heat transfer enhancement methods to helically coiled tube. Moreover, the subject of single-phase flow in helically coiled tubes is ascertained to be worth researching due to the fact that there are limited number of studies and is still no empirical or analytical model/correlation in the case of using enhanced surfaces and wire coil insert. Forthcoming researches on this issue in the near future will be considered as pioneer ones in literature.

Heat transfer enhancement for U-pipe evacuated tube solar absorber by high-emissivity coating on metal fin

In the solar thermal field, the U-pipe evacuated tube collector has been widely used to provide hot water for domestic usage and space heating, or integrated with concentrating reflector to deliver thermal power in higher temperature range. Enhancing heat transfer process inside the U-pipe evacuated tube has significant effect on improving the overall thermal efficiency of the collector. However, the current heat transfer enhancement methods require either structure modification or material property change, and are therefore difficult to be promoted for commercial use. Herein, we proposed an easy and feasible method of depositing a high-emissivity coating on a metal fin to enhance the interior heat transfer. The effect of applying this method on the U-pipe evacuated tube with and without compound parabolic concentrator integration was investigated by numerical simulation. We found that this method exerts elevating improvement effect with an increasing operating temperature of the collector. The proposed method yields a maximum improvement of 2.7% for a single U-pipe evacuated tube operating up to 150 °C. Higher optimization effect was achieved in the case of compound parabolic concentrator integration. By employing this method, we found an 8.4% performance improvement when the tube is combined with a concentrator with concentration ratio of 6.0 at an operation temperature of 200 °C. At the background that the structure of the U-pipe has been mature and commercialized for a long time, the proposed method with easy-feasibility and low cost is expected to have potential for implementation.

Thermal performance and entropy generation for nanofluid jet injection on a ribbed microchannel with oscillating heat flux: Investigation of the first and second laws of thermodynamics

In current numerical study, forced flow and heat transfer of water/NDG (Nitrogen-doped graphene) nanofluid in nanoparticles mass fractions ( φ ) of 0, 2% and 4% at Reynolds numbers ( Re ) of 10, 50, 100 and 150 are simulated in steady states. Studied geometry is a two-dimensional microchannel under the influence of nanofluid jet injection. Temperature of inlet fluid equals with T c = 293 K and hot source of microchannel is under the influence of oscillating heat flux. Also, in this research, the effect of the variations of attack angle of triangular rib (15°, 30°, 45° and 60°) on laminar nanofluid flow behavior inside the studied rectangular geometry with the ratio of L / H = 28 and nanofluid jet injection is investigated. Obtained results indicate that the increase of Reynolds number, nanoparticles mass fraction and attack angle of rib leads to the increase of pressure drop. By increasing fluid viscosity, momentum depreciation of fluid in collusion with microchannel surfaces enhances. Also, the increase of attack angle of rib at higher Reynolds numbers has a great effect on this coefficient. At low Reynolds numbers, due to slow motion of fluid, variations of attack angle of rib, especially in angles of 30°, 45° and 60° are almost similar. By increasing fluid velocity, the effect of the variations of attack angle on pressure drop becomes significant and pressure drop figures act differently. In general, by using heat transfer enhancement methods in studied geometry, heat transfer increases almost 25 %.

A Review of Recent Passive Heat Transfer Enhancement Methods

Improvements in miniaturization and boosting the thermal performance of energy conservation systems call for innovative techniques to enhance heat transfer. Heat transfer enhancement methods have attracted a great deal of attention in the industrial sector due to their ability to provide energy savings, encourage the proper use of energy sources, and increase the economic efficiency of thermal systems. These methods are categorized into active, passive, and compound techniques. This article reviews recent passive heat transfer enhancement techniques, since they are reliable, cost-effective, and they do not require any extra power to promote the energy conversion systems’ thermal efficiency when compared to the active methods. In the passive approaches, various components are applied to the heat transfer/working fluid flow path to improve the heat transfer rate. The passive heat transfer enhancement methods studied in this article include inserts (twisted tapes, conical strips, baffles, winglets), extended surfaces (fins), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), and nanofluids (mono and hybrid nanofluids).

Transient Cooling Performance of R134a Flash-Evaporation Spray Enhanced by Cold Air Jet: A Novel Heat Transfer Enhancement Method in Laser Dermatology

Transient Cooling Performance of R134a Flash-Evaporation Spray Enhanced by Cold Air Jet: A Novel Heat Transfer Enhancement Method in Laser Dermatology

A critical review on thermal management technologies for motors in electric cars

• Advanced thermal management technologies for motors in electric cars are reviewed. • Air and liquid cooling methods for stator, winding and rotor were critically examined. • Possible methods to enhance the heat conduction in motors are thoroughly evaluated. • Pros and cons of current studies are presented, with suggestions for future work. The development of electric cars has well been regarded as a major solution for tackling the challenges of carbon-neutrality faced by the modern communities. Electric motor is certainly the core and most important components of an electric car, and the thermal management for electric motors has drawn increasing attention from both industry and academic society. This is because electric motors in modern electric cars are required to be more powerful and competitive with higher torque, higher speed and higher power density, therefore the efficient thermal management has become essential to maintain the motors efficiency, durability and safety. The failure of thermal management will result in demagnetization of magnets, aging of the insulation materials, decrease of efficiency, shorter lifetime and even burnout of motors. To enlighten the future research, in this paper, both the theoretical modeling and experimental investigations of the latest thermal management methods are reviewed. The state-of-the-art of various thermal management techniques, including air cooling (natural and forced air cooling, air impingement cooling) and liquid cooling (water/oil jacket cooling, jet impingement cooling, spray cooling, immersion cooling, slot channel forced convection cooling) for the stator, winding and rotor are critically presented. Meanwhile, heat transfer enhancement methods by conduction based on potting materials, thermal paste, heat guides, PCMs and heat pipes are highlighted. Following that, hybrid thermal management technologies to address extreme conditions are also discussed. In the last section, some suggestions are given for future possible research and applications. The paper is expected to be a good reference and inspiration for the development of new thermal management concepts of electric motors.

Experimental performance of a solar air heater using straight and spiral absorber tubes with thermal energy storage

A flat plate solar air collector is a simple energy conversion system with relatively poor performance due to low heat transfer between air and absorber. Passive heat transfer enhancement methods increase the absorber-side contact area and the localized air turbulence. This research article introduces a dual-purpose copper tube as an extended absorber surface as well as heat storage. The absorber designs are spiral coiled and straight tubes with uniform spacing, filled with glycerol and commercial grade paraffin. Thermal and exergy efficiencies of modified solar air heater configurations are compared with conventional collector with a plain absorber without energy storage. The experimental results show that the double-pass solar collector with a spiral tube with paraffin wax has higher thermal performance than the straight tube and conventional collectors. The spiral coil's thermal and exergy efficiencies with paraffin wax are 29.86% and 1.11%, respectively, at an airflow rate of 0.0076 kg/s with a maximum attained air temperature of 59 °C. The air temperature is maintained for a short time in the heat storage-based absorber even after sunset. Thus, the integration of thermal storage into the solar collector stores the heat for off-sunshine hours and enhances the absorber's energy storage capacity and uniform temperature. The modified absorber surface with heat storage results in better performance without auxiliary power. Further, the enviro-economic study ensures a lower payback period for PCM-integrated spiral tube collectors of about 2.4 years. Such simple and economic absorber designs are beneficial to various solar drying and space heating applications.

Visualization and Sound Measurements of Vibration Plate in a Boiling Bubble Resonator

We developed a boiling bubble resonator (BBR) as a new heat transfer enhancement method aided by boiling bubbles. The BBR is a passive device that operates under its own bubble pressure and therefore does not require an electrical source. In the present study, high-speed visualization of the flow motion of the microbubbles spouted from a vibration plate and the plate motion in the BBR was carried out using high-speed LED lighting and high-speed cameras; the sounds in the boiling chamber were simultaneously captured using a hydrophone. The peak point in the spectrum of the motion of the vibration plate and the peak point in the spectrum of the boiling sound were found to be matched near a critical heat-flux state. Therefore, we found that it is important to match the BBR vibration frequency to the condensation cycle of the boiling bubble as its own design specification for the BBR.

Aerothermal performance of a rotating two-pass furrowed channel roughened by angled ribs

• A compound-HTE method with angled ribs and furrowed passages was devised for cooling of gas turbine blades. • Without rotation, Nu 0 were raised to 4.9–4.77 times D-B levels with f 0 between 33.15 and 35.92 times Blasius equation values. • With rotation, overall Nu ( f ) on leading and trailing walls were 1.03–2.0 (1.10–1.66) and 1.02–1.91 (1.12–1.74) times Nu 0 ( f 0 ). • Thermal-performance indexes of static (rotating) channels were 1.53–1.45 (2.39–1.48). The aerothermal performance of a novel compound heat transfer enhancement method is assessed by measuring the full-field endwall Nusselt number map and Fanning friction factors of a rotating two-pass furrowed channel roughened by angled ribs. The tested Reynolds numbers, rotation numbers, and buoyancy numbers lie in the respective ranges of 5000–15,000, 0–0.4, and 0–0.183. Under the combined effects of the ribs and furrowed passages, the endwall-average Nusselt numbers of the static channel are upraised to 4.9–4.77 times the Dittus–Boelter levels. With rotation, the worst cooling condition develops at a rotation number of 0.05 with the spotted heat-transfer impediments to 0.77 and 0.82 times the static channel references on the leading and trailing endwalls, respectively. By raising the rotation number from 0 to 0.4, the Nusselt numbers on the stable endwalls are initially reduced from the static-channel levels and then converted to increase with the rotation number; however, the friction factors increase continuously. Over the rotating unstable endwalls, both the Nusselt numbers and the friction factors increase along with rotation number. When the buoyancy number is increased, the local Nusselt numbers on the leading and trailing endwalls increase with a negligible impact on their distribution patterns. The thermal-performance indexes of the rotating channel are reduced by increasing Reynolds number. To aid the relevant applications, two sets of experimental correlations are devised to determine the regional average Nusselt number and Fanning friction factor of the rotating two-pass furrowed channel fitted with angled ribs.

Proposing an innovative and explicit economic criterion for all passive heat transfer enhancement techniques of heat exchangers

Numerous passive heat transfer enhancement techniques (including various types of turbulators) have been proposed before for heat exchangers by many researchers. Their thermal/frictional behaviors have been reported in-detail in term of Nu number, friction factor and so on. However, their economic characteristic has been always immature which is because of lack of an explicit economic criterion (with clear economic unit i.e. dollar per unit of time etc.) applicable for any passive technique through any type of heat exchanger. Hence, this research aims to propose an explicit economic criteria (with a final clear formula) to evaluate the production cost rate of heated/cooled fluid through any type of heat exchanger (with or without passive technique) taking into account all effective parameters (such as capital cost, pumping power, exergy related costs, electricity price of the region, thermal and fluid flow condition through the heat exchanger, ambient condition and so on) and without dependency of other equipment that may work in-line with heat exchanger. The model is developed based on the general standard Specific Exergy Costing theory. The proposed model is a strong economic criterion tool, optimization tool and also comparison tool between different passive heat transfer enhancement methods. Case study as an example application of the model is provided at the last part of the paper.

Development of Methods for Heat Transfer Enhancement During Nitrogen Boiling to Ensure Stabilization of HTS Devices

Development of Methods for Heat Transfer Enhancement During Nitrogen Boiling to Ensure Stabilization of HTS Devices

Performance enhancement of a maple leaf-shaped latent heat energy storage unit by adding nanoparticles and leaf vein fins

In this paper, the solidification process of a phase change material (PCM) in a maple leaf-shaped Latent Heat Energy Storage (LHES) system applying fins in the form of leaf veins and nanoparticles as direct and indirect heat transfer enhancement methods is investigated numerically. These systems are used to balance energy supply and need. In this research, both methods are used to solve the problem of pure PCM, ie low thermal conductivity. The innovative fins used in this work are geometrically optimized using the Response Surface Method (RSM). The goal of optimization is accelerating the solidification process and reducing the full solidification time (FST). In the second method, nano-enhanced phase change material (NEPCM) is made by adding MWCNT nanoparticles to PCM (water). Standard Galerkin Finite Element Method (SGFEM), along with adaptive grid refinement, has been used to simulate the solidification process. The two methods were examined separately and simultaneously (Hybrid method), and the results were compared. Although the hybrid use of the two aforementioned methods is excellent with a 56.9% reduction in FST if the two methods are used separately, the method of using optimized fins in the form of leaf veins with a 42.6% reduction in FST is much more effective than making NEPCM with a 25.5% reduction in FST.

Resonance-driven heat transfer enhancement in a natural convection boundary layer perturbed by a moderate impinging jet

We report a convective heat transfer enhancement driven by flow resonance downstream of a natural convection boundary layer that is perturbed by a moderate impinging jet. Our experimental observations and large eddy simulations confirm flow resonance between the thermal boundary layer and a periodic flow at the jet-impinging region. Downstream heat transfer enhancement from the baseline of natural convection exceeds 40%. The results support the development of efficient heat transfer enhancement methods.

Heat transfer improvement in a tube by inserting perforated conical ring and wire coil as turbulators

AbstractIn the present study, two various passive methods for heat transfer enhancement, including conical ring and wire coil are placed in a tube as turbulators. Four conical rings with four side holes are utilized with the same distance. The wire coil is employed at the center of the tube. The considered Reynolds numbers are between 4000 and 10,000. The studied geometrical parameters contain the pitch and diameter of a wire coil. Four different pitches of wire coil, including 10, 12, 14, and 16 mm, are evaluated. Furthermore, four values of wire coil diameter such as 2, 4, 6, and 8 mm are certain. The obtained numerical results displayed that by declining the pitch of a wire coil (37.5%), the average Nusselt number increases by about 143%. Also, augmentation in wire coil diameter by 300% leads to a growth in average Nusselt number by about 131%. Moreover, owing to utilizing two various turbulators, the pressure drop is significantly high in comparison with the bare tube. At Re = 10,000, growth in the inner diameter of the wire coil by 300% leads to an increase in thermal performance by about 36.12%. Moreover, as the pitch of the wire coil rises by 60%, the thermal performance declines by about 35.71%.

Effect of pitching and rolling motion on hydrothermal performance of rectangular channel flow enhanced by twisted-tape pin–fin array

Abstract Motivated by the requirements for the effective heat exchange with marine applications, the hydrothermal performances of the rectangular channel enhanced by the array of twisted-tape pin-fins in rolling and pitching motions were studied using the steady-state infrared thermography method. The distributions of endwall Nusselt number, the Fanning friction coefficients, and the thermal performance factors were measured at Reynolds numbers of 1500, 2500, 3500, 4500 and 5500 with the test channel in single pitching, single rolling and combined pitching-rolling motions. For each mode of swinging condition with the frequency of 0, 0.5, 0.667 or 0.833 Hz at each Reynolds number tested, four heater powers were applied to the test channel for exploring the buoyancy effect on the heat transfer performance. While the present pitching or rolling forces agitated the vortical flows developed in the pin–fin channel to augment strain rates for heat transfer enhancements, the synergetic heat transfer impact at the combined pitching-rolling mode suppressed the effectiveness of heat transfer enhancement attributed to the single pitching and single rolling motions. Relative to the static-channel conditions, the time-mean averaged Nusselt numbers in the single pitching, the single rolling and the combined pitching-rolling motions were modified to 0.88–1.31, 1.02–1.55 and 0.98–1.77 times of the static-channel Nusselt numbers with the corresponding elevations of friction factor to 1.05–2.59, 0.76–2.50 and 2.09–6.79 times of the static-channel references. At all the swinging conditions tested, the time-mean local and area-averaged Nusselt numbers were increased by enhancing the buoyancy level. The considerable pressured drop augmentations at the combined pitching and rolling modes led to the degradation of the hydrothermal performance from that with the single pitching or the single rolling motions. In addition to the newly devised array of twisted tapes as a passive heat transfer enhancement method, the individual and synergetic impacts of pitching and rolling motions on the detailed heat transfer distribution revealed by the present study went beyond the previous efforts in the literature. For assisting the relevant engineering applications, two set of empirical correlations that calculated the time-mean endwall averaged Nusselt number and Fanning friction coefficient of the present twisted-tape pin–fin channel in single pitching, single rolling and combined pitching-rolling motions were devised.