Heat Transfer Enhancement Techniques Research Articles

Numerical investigation and extensive parametric analysis of cryogenic latent heat shell and tube thermal energy storage system

• Cryogenic latent heat thermal energy storage for boil-off gas reliquefaction. • Phase change numerical modelling using the enthalpy method application on Dymola. • Parametric study on the system geometric design and the fluid mass flow rate. • Study of radial and longitudinal fins as heat transfer enhancement techniques. • Performance evaluation using charging/discharging rates and energy storage data. Latent heat thermal energy storage systems have shown promising energy management solutions in different domains due to their high efficiency and energy density. Nowadays, most studies focus on moderate to high operating temperature applications such as solar power plants, while few studies exist for cryogenic applications such as the liquefied natural gas (LNG) domain. This article aims to assess the thermal performance of a shell and tube latent heat thermal energy storage system. The purpose of this system is to reliquefy the excess boil-off gas (BOG) generated on LNG carriers, using n-Butane as a suitable phase change material (PCM). The system is numerically investigated using Dymola software, where the solidification and melting evolution of the PCM is accounted for by using the enthalpy method and applying the finite volume method discretization technique. This system works on two operating modes: charging and discharging. During charging, the PCM solidifies and stores the cold energy during the LNG evaporation to be discharged at later times to condense the BOG while the PCM melts. An extensive parametric study is carried out to assess the impact of different factors on the system’s thermal performance. Results showed that the existence of natural convection would significantly enhance the PCM performance. Moreover, the selection of the tube diameter, PCM thickness, and the heat transfer fluid mass flow rate is crucial and is a compromise of several factors. On the other hand, radial and longitudinal fins were assessed as heat transfer enhancement techniques, where a parametric study on the fins’ volume, thickness, and number is carried out. The excessive addition of fins is expected to obstruct the natural convection forces; however, a considerable enhancement in the thermal performance, especially for the radial ones, is observed.

A recent state of art review on heat transfer enhanced characteristics and material selection of SCTHX

The shell and coil tube heat exchangers (SCTHX) are widely used to transfer heat energy from one medium to another effecting medium, such as refrigeration and air conditioning system, chemical reactor, solar heater and steam power plant. The helical tubes are one of the passive heat transfer enhancement technique in heat exchanger, which are adopted in industries due to higher rate of heat transfer and simplicity in manufacturing. It has been widely employed that heat transfer coefficient in helical coils are higher compare to straight pipe by several researchers. The reason is behind that the formation of secondary flow of nano fluid. It is leading by dean vortex which is imposed to primary flow. Furthermore research on segmental and helical baffle geometry can improve the heat transfer rate in helical coil tube heat exchanger. This study is concerned with the enhancement of convective heat transfer interaction between fluid and solid surface of helical coil. The detailed descriptions of material selection, nano fluid flow physics and heat transfer enhancement inside helical coil tube are very less in open literature. A state of art review indicates that further research work can be carried out by modifying the geometry of SCTHX and enhance factors of fluid property. The heat transfer can also be enhanced by polymer additives added in nano fluids, effect of super hydrophobic surfaces and fluid bubbles generation from roughened heating surface. Finally new ideas are presented in this paper to improve the heat transfer rate and material selection for low cost manufacturing with minimum size.

Experimental and numerical optimization of cascaded PCM heat sink by using low melting point alloys

The effectiveness of cascaded phase change materials (PCMs) in improving the efficiency of latent heat storage systems has been widely proved. However, this heat transfer enhancement technique is seldom applied in the field of PCM-based thermal management. To improve the thermal management efficiency of PCM heat sink, a novel cascaded PCM heat sink with low melting point alloys is proposed. Firstly, three single PCM heat sinks and three cascaded PCM heat sinks are experimentally tested within a heat flux range of 1.0 to 3.0 W/cm2. Then, a simplified one-dimensional model is established to further analyze the enhancement mechanism of the cascaded PCMs. At last, the volume fraction of each PCM in the cascaded PCM heat sink is optimized by coupling the genetic algorithm and the numerical model. The experimental results prove that the application of the cascaded PCMs can improve the thermal management performance of the PCM heat sink effectively. Under the heat flux condition of 3.0 W/cm2, the maximum managed time of the cascaded PCM heat sinks is about 8.47 min, which is 7.2 % longer than that of the single PCM heat sinks. The configuration and volume fractions of PCM have a significant influence on the thermal performance of the cascaded PCM heat sinks. Though optimizing the volume fractions of each PCM, the managed time of the optimized cascaded PCM heat sink is further prolonged to about 8.88 min. The current study not only exhibits the application potential of the cascaded PCMs in electronic component cooling, but also provides guidelines for the design and implementation of cascaded PCM cooling systems for other cooling applications.

Review on Boiling Heat Transfer Enhancement Techniques

Boiling is considered an important mode of heat transfer (HT) enhancement and has several industrial cooling applications. Boiling has the potential to minimize energy losses from HT devices, compared with other convection or conduction modes of HT enhancement. The purpose of this review article was to analyze, discuss, and compare existing research on boiling heat transfer enhancement techniques from the last few decades. We sought to understand the effect of nucleation sites on plain and curved surfaces and on HT enhancement, to suggest future guidelines for researchers to consider. This would help both research and industry communities to determine the best surface structure and surface manufacturing technique for a particular fluid. We discuss pool boiling HT enhancement, and present conclusions and recommendations for future research.

Numerical study on heat transfer enhancement by viscoelastic fluid pulsating laminar flow in rectangular microchannel heat sinks

Efficient heat transfer technique at microscale is a long-term demand for development of microelectronic device. Simple-structured microchannel heat sinks (MHSs) have the advantages of easier fabrication processes and profound applicability, which are however limited by the laminar flow with lower heat transfer efficiency. Against this backdrop, a new type of heat transfer enhancement (HTE) techniques by viscoelastic fluid pulsating laminar flow (VPL flow) is proposed and evaluated from different aspects in this research. First of all, the effects of VPL flow on the thermal performance in a rectangular microchannel heat sink (RMHS) are numerically studied with the focus on the parametric effects on HTE. It indicates that the overall heat transfer performance can be significantly augmented by VPL flow and possesses an optimal enhancement under cases of exerted pulsation frequencies closed to the characteristic frequency of viscoelastic fluid flow. The characteristic frequency is determined from the Poiseuille start-up flow of the viscoelastic fluid flow and can be summarized by a power-law function with the relationship of Reynold number (Re) and Elasticity number (El). Moreover, the optimal HTE of RMHS increases with external pulsation amplitude and El but decreases with Re, which can also be summed up with them in a power-law function. Then, by analyzing of flow performance, the reversal flow and the resonance effects by VPL flow are regarded as the reasons for the HTE, which results from a balance between relaxation time of viscoelastic microstructures deformation and external pulsation period. Finally, the law of HTE by VPL flow is extended and verified in a carefully designed leaf-type microchannel heat sink (LMHS), suggesting a promising micro-scale heat transfer enhancement technology by VPL flow which however still needs further investigations on the optimal operating conditions in the future.

(Digital Presentation) Heat Transfer Enhancement in Room-Temperature Metal Hydride Storage Systems

Among the existing methods of short- and long-term storage of renewable energy, reversible metal hydrides (MH) are considered safe and volume efficient. They provide efficient hydrogen storage capacity for various applications, including Low Temperature PEM Fuel Cells (LT PEMFCs) [1]. The heat transfer in the MH reactor has a substantial influence on the efficiency of the storage process. In order to achieve the established goals for tank filling times, enhanced heat transfer techniques will be essential [2]. Therefore, this work investigates technical solutions for thermal conductivity enhancement in MH storage systems. Different tank designs are assessed in terms of their applicability and the effect of different solid matrices in the storage system on the internal heat transfer is studied. With the help of thermal imaging, the impact of the geometry of the solid matrices on the uniformity of gas distribution along the MH bed is analysed. The MH tanks are operated with room temperature (RT) metal hydrides that achieve a hydrogen storage capacity of up to 105 kgH2 m−3 in a wide range of temperatures (from 50 to -40°C). The tests are carried out within a limited pressure range of 1 to 0.1 MPa to reduce the reliance on additional H2 compressors when used in stationary applications.[1] M. V. Lototskyy et al., „Metal hydride systems for hydrogen storage and supply for stationary and automotive low temperature PEM fuel cell power modules“, Int. J. Hydrog. Energy, Bd. 40, Nr. 35, S. 11491–11497, Sep. 2015.[2] K. C. Smith, Y. Zheng, T. S. Fisher, T. L. Pourpoint, und I. Mudawar, „Heat Transfer in High-Pressure Metal Hydride Systems“, J. Enhanc. Heat Transf., Bd. 16, Nr. 2, S. 189–203, 2009.

Multi-objective optimization of a bidirectional-ribbed microchannel based on CFD and NSGA-II genetic algorithm

The bidirectional rib (BR) has been recognized as one effective heat transfer enhancement technique for the microchannel heat sink (MCHS) since it can realize the heat transfer improvement in both vertical and spanwise directions. However, the utilizing of BR causes a large pressure drop penalty in the microchannel due to its broader obstruction area. For the engineering application, the heat transfer and pressure drop penalty of MCHS are two key metrics to evaluate its overall performance. To address this optimization problem, 144 sample points of the microchannels with different BRs are generated by CFD simulation, and the NSGA-II has been utilized to search for the optimal solutions in-terms of both objectives. For most optimal solutions, the length of SR and rib pitch are 200 μm and 800 μm, respectively, while the other two parameters, the width and height of SR, almost cover the entire value range. Subsequently, the best compromise solutions have been determined from the Pareto fronts by the TOPSIS and LINMAP methods according to the decision maker's preference. Finally, the selected optimum solutions are verified by numerical and experimental methods.

A Comprehensive Review and Comparative Study of Various Rib Configurations Used in Rectangular Duct for Heat Transfer Enhancement

This research study provides a comprehensive review of various heat transfer enhancement techniques that are being used by various researchers for enhancing heat transfer rate and overall thermal performance factor of the heat exchangers. Based on the present review study, it can be concluded that intensive study has been carried out by various researchers to determine the effect of various rib geometries on the heat transfer enhancement and friction factor characteristics of rectangular duct. The enhancement in the heat transfer rate is obtained by introducing the blockages or restrictions in the fluid flow path by providing various rib turbulator configurations. This research paper puts highlights on the various heat transfer enhancement techniques are used for increasing overall thermal performance factor in the heat exchangers. Introduction of ribs as flow disturber results not only into increase in heat transfer surface area but also into increased turbulence, recirculation of working media, viscous effect and secondary flow that results into swirl. The interruptions or blockages created by ribs provided in the flow channel results into increased pressure drop and disruptions in boundary layer development. Provision of ribs also causes secondary flow which ensures enhanced thermal contact between working fluid and surfaces of the flow channel. This results into increase in heat transfer coefficient and thermal performance of heat exchanger respectively.

Numerical investigation of pentagonal V-shape ribs to enhance heat transfer in hydrogen rocket engine cooling channels

Keeping the combustion chamber walls at a sufficiently low temperature ensures liquid rocket engines' safe and reliable operation. The most promising method to achieve that is the heat transfer enhancement techniques which optimize the heat transfer coefficient and increase the thermo-hydraulic performance. The present work is concerned with numerical studies on the effect of the pentagonal V-shape ribs to enhance the heat transfer coefficient of the rocket engine's cooling channels. Numerical simulations for various chamfering angels of the pentagonal V-shape rib are performed by the (SST,k−ω) turbulent model. The simulation findings show that the overall performance of the ribbed channel improved inversely with the pentagonal chamfering angle. Compared with a conventional channel, the average Nusselt Number and thermal-hydraulic performance of the ribbed channel enhanced up to 66.6% and 52% respectively, and the 5° pentagonal V-shape rib has higher thermal-hydraulic performance with a slight high the pressure loss penalty.

Effects of design parameters on flow fields and heat transfer characteristics in semicircle oblique-finned corrugated

In this study, a new technique for heat transfer enhancement through the introduction and analysis of a semicircle oblique-finned corrugated has been studied numerically. Different design parameters are employed, including the vertical gap ratio (H) of 0.03, 0.04, and 0.05, the horizontal gap ratio (V) of 0.15, 0.20, and 0.25, as well as oblique-angle (θ) of 10°, 20°, and 40° are examined. Finite volume method (FVM) and the κ-ε model are used to simulate turbulent air flows inside the targeted channels at constant temperature conditions (T = 300 K) over Reynolds number between 10,000–50,000. The findings indicate that the employment of the new technique resulted in increased heat transfer and, as a consequence, increased thermal-hydraulic performance (PEC) via improved fluid mixing. The results also indicate that the effect of frictional loss has been used to influence the target system's overall performance in the direction of minimising the effect of heat transfer. The results show that a small vertical gap ratio and a high horizontal gap ratio have a greater effect on both heat transfer rate and PEC than others. Generally, the best PEC of 2.469 has been observed when V = 0.15, H = 0.05, and = 40 are used as design parameters. In terms of Nusselt number and friction factor, new correlations for the new model of semicircle oblique-finned corrugated are also reported.

Role of phase change materials in thermal energy storage: Potential, recent progress and technical challenges

Thermal energy storage (TES) using phase change materials (PCM) have become promising solutions in addressing the energy fluctuation problem specifically in solar energy. However, the thermal conductivity of PCM is too low, which hinders TES and heat transfer rate. In recent days thermally enhanced PCMs are a promising candidate for TES and heat transfer applications. This research designates the review on various thermal conductivity and heat transfer enhancement techniques of PCMs like, increasing the heat transfer area, use of various highly conductive porous foam materials (copper, aluminium, nickel, graphite, and carbon), mix of materials, use of multiple PCM and nano dispersed PCM (Copper oxide (CuO), Aluminium dioxide (Al2O3), Carbon nanotubes (CNTs), graphene, etc.). Also, the long-term stability, phase segregation and super colling are extensively discussed. Furthermore, energy storage applications of highly conductive PCMs in advanced fields like solar thermal system, desalination units, pharmaceutical industry, fabrics, food processing, battery thermal management etc are discussed. It has been observed that porous materials/foam dispersed PCM had better heat transfer/storage capacity (thermal conductivity 2–500 times more). In addition, organic PCMs have been widely used, due to its long-term stability, low or no supercooling, less or no corrosion, and recyclability. The highly stable thermally enhanced PCMs are used for long-term TES/heat transfer applications.

Heat Transfer Enhancement of Ionic Wind Assisted Slot Jet Reattachment Nozzle: A Numerical Study

Abstract Impinging jet nozzles are omnipresent in industrial applications. Innovative impinging jet nozzles, such as radial jet reattachment (RJR) and slot jet reattachment (SJR) nozzles, have been proven to be highly efficient tools to enhance heat and mass transfer, compared to traditional in-line jet and slot-jet nozzles. However, the heat and mass transfer in the region immediately underneath these reattachment nozzles are relatively inefficient. The ionic wind is a promising technique for heat transfer enhancement. The ionic flow is induced when free ions are accelerated by an electric field, and exchange momentum with neutral air molecules. In this numerical study, the performance of the SJR nozzle is improved by the application of an electric field, specifically, ionic wind, which is generated in the region directly between the nozzle and the exposed impingement surface. The two-dimensional numerical model is based on the flow field generated by an ionic wind-assisted SJR nozzle. The simulation results show a significant secondary flow induced under the nozzle, due to ionic wind. A significant enhancement of local and average heat transfer coefficients is achieved. The heat transfer increases with the applied potential and nozzle exit velocity. However, the SJR flow pattern is altered when the air exit velocity is below a certain threshold. The simulation results provide an in-depth understanding of the heat transfer characteristics under various operating conditions and pave the way for developing this novel impinging nozzle design.

Effects of 25 kHz ultrasound from single and double transducers on thermal and friction characteristics of laminar flows in water or water-based Al2O3 nanofluids

The heat transfer and flow resistance were investigated experimentally in a rectangular duct based on compound techniques of heat transfer enhancement using 25 kHz ultrasound and Al2O3-water nanofluid. The experiment was conducted at flow rates of 0.5–2.0 l/min under constant heat flux and laminar flow conditions. Two ultrasonic transducers were mounted on the top wall of the test section to release the waves from single or double transducers in a downward direction, perpendicular to the mainstream flow. Water and Al2O3(nanoparticle volume fraction (φ) = 0.8–6.1%) nanofluid were utilized as different test fluids. The results showed that the surface temperature increased with the volume fraction of Al2O3, and rapidly reduced after applying the ultrasonic waves. At the same flow rate, the peak augmentation of the local Nusselt number using ultrasound from a single transducer was 1.31 in Al2O3(φ = 6.1%), which increased to 1.58 using double transducers in Al2O3(φ = 1.5%). The flow resistance was increased by adding the Al2O3 nanoparticles, with increased resistance with an increased volume fraction. The friction loss increased up to 3 times for φ = 6.1% compared to the base fluid of water, while it slightly increased approximately 1–2% using ultrasound. These results indicated that using these techniques, the thermal performance tended to increase with a decreased Reynolds number and volume fraction. Finally, predictive formulas for the Nusselt number and friction factor were developed for using single or double ultrasonic transducers in water or Al2O3(φ = 0.8–6.1%) nanofluids under a laminar flow regime.

Review on Nano-Fluids Applications and Heat Transfer Enhancement Techniques in Different Enclosures

Nano-fluid applications span such a broad range of topics in the practical field that they demand their own review articles. In this review paper, the use of magnetic fields, porous media, and Nano-fluids in different heat transfer applications is discussed mainly in the solar thermal field. It has been proven that the employment of these techniques provides significant enhancement results for convective flows especially when they are combined, also the mathematical equations used to model this type of flow are summarized. In addition, different studies reported that the geometrical parameters of the enclosures can also effect the flow. In this context, recently scholars maintained many investigations on complex shaped cavities and their impact on heat transfer. These studies showed promising results for the use of this type of geometries especially for the trapezoidal ones. As reviewed in this paper, trapezoidal geometries and their properties strongly effect the convective flow in a great way leading to considerable enhancement. Overall, this review aims to present an insightful vision on different heat transfer improvement techniques and values the use of these methods in trapezoidal geometries for solar heat transfer applications.

Design and test of shape memory alloy fins for self-adaptive liquid cooling device

• Self-adaptive cooling fins based on Shape Memory Alloys (SMA). • The set of conditions for proper operation are defined analytically. • The impact of an array of SMA fins on thermal resistance is experimentally studied. • Nearly constant wall temperature is achieved for varying heat flux scenarios. Thermal management complexity increases in high-performance chips, where the heat loads vary spatially and temporally, while liquid cooling systems are usually designed for most stringent stationary conditions. Several works developed heat transfer enhancement techniques to increase the cooling capacity of liquid cooled heat sinks, but pumping power is increased in a permanent way due to the addition of elements within the channels. Here, a liquid cooling self-adaptive heat sink that can efficiently adapt the distribution of its heat extraction capacity to time dependent and non-uniform heat load scenarios is proposed. Numerical design of the mesoscale cooling device with bimorph metal/SMA fins, definition of the fabrication and training procedure of the SMA fins to reach the desired behavior and experimental assessment is presented. The capacity of the self-adaptive fins to locally boost the heat transfer is numerically and experimentally demonstrated. Results obtained show that the self-adaptive fins can improve the temperature uniformity by 63% with respect to plain channel. The reduction in thermal resistance using bimorph metal/SMA fins sample allows the surface maximum temperature gradient to remain almost constant although heat flux increases. Energy savings are maximized in applications where partial load intervals contributes significantly to the overall operating period.

Maximum benefits from the use of enhanced heat transfer surfaces

The paper presents a critical review of the performance evaluation criteria (PEC) developed by Webb (Webb, 1981). These criteria evaluate the benefits that could be obtained in heat exchangers if their surfaces and inserts are designed to enhance the heat transfer, particularly in two-fluid heat exchangers in single-phase flow. This study revealed that the imposed constraint for fixed pumping power in the comparison of the augmented channel with the reference one is a major obstacle to achieving greater benefit. Accordingly, the elimination of the constraint of fixed pumping power and the objective of reduced pumping power decreases substantially the criteria of Webb. A new constraint related to the fixed augmentation entropy generation number has been involved replacing the constraint of fixed pumping power. The pumping power can be left to increase until the augmentation entropy generation number is less than unity. The maximum benefit can be obtained when the augmentation entropy generation number reaches the value of unity. The use of greater in tube heat transfer enhancement (with the use of compound heat transfer enhancement techniques instead of simple ones), and with an enhanced outer tube surface of the augmented exchanger is a useful instrument to increase the benefits, only if the augmentation entropy generation number does not cross the frontier of unity.

Melting of hybrid nano-enhanced phase change material in an inclined finned rectangular cavity for cold energy storage

The aim of this paper is to study the potential of two heat transfer enhancement techniques, namely adding fins and hybrid nanoparticle (Cu and Al2O3), for improvement of the melting process of PCM (water) in an inclined rectangular enclosure as model of cold energy storage systems. Phase change and heat transfer of PCM are modelled using an in-house built code based on Lattice Boltzmann Method, which is validated with experimental and numerical results from literature. Enhancement effect is investigated through the main parameters including number of fins, fin length ratio (W/H) and nanoparticle volume fraction (ϕ). Heat transfer and fluid flow, liquid fraction, transient evolution of melting front, required time for full melting, and thermal energy storage are analysed in detail. The results show that the melting rate is about two times higher for the fin length ratio of W/H = 0.75, compared to W/H = 0.25. Besides, nanoparticle loading further enhances the melting rate. The highest melting rate is attained in the case of 3-fins and nanoparticle concentration of ϕ = 6 vol% with a 33.5% reduction in the complete melting time. Incrementing the fin length ratio increases the thermal energy stored, and the melting time can be shortened by up to 64%.

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).

A FIELD SYNERGY PRINCIPLE OF VELOCITY AND PRESSURE FOR FLOW RESISTANCE REDUCTION

Both the heat transfer and the flow resistance are important for the design of heat transfer enhancement techniques. The field synergy principle of velocity and temperature has been widely used in the optimization of convective heat transfer processes, but the field synergy principle for flow resistance is still lacking. In this work, a field synergy principle between velocity and pressure is derived for the flow resistance. The derivation of the principle is based on the divergence theorem. The differential equations are used to clarify the physical meaning of the principle. The analysis demonstrates that the negative of the integral of the product of velocity and total pressure gradient equals the pumping power of the flow. Therefore, the principle states that the better the synergy of velocity and total pressure fields, which means smaller intersection angle between velocity and total pressure gradient, the lower the pumping power consumed. The static pressure can also be used in the principle when the change of the flow rate of dynamic pressure is insignificant. Numerical examples are used to validate the principle. This study indicates that the synergy among velocity, temperature, and pressure fields is important for the design of high-efficiency heat transfer enhancement techniques.

Experimental analysis of copper coated helical coiled tube heat exchanger using nanofluid and water

Many studies focused on improving heat transfer by increasing the electrical, industrial, and transportation processes that are required to achieve a higher heat transfer rate. In general, there are two types of heat transfer enhancement techniques: active and passive. External power inputs such as pulse flow, vibratory force, and a magnetic field are used in the active approach. The heat transfer rate is increased upto 20 % in the passive technique by adding more devices or changing the heat transfer surface. The nanoparticle suspension is a heat transfer improvement approach in this study. An experimental investigation used Al2O3-water nanofluid in the counter flow heat exchanger to improve heat transfer in the range of 20 %. Experiments with varied volume concentrations 0.1 and 0.15 and mass flow rates of nanofluid were carried out. The arrangements are utilized to assess the situation. The temperature fluctuations were measured using the setups and the acquired values are compared to prior nanofluid performance results, and the result demonstrates that the heat transfer rate has risen upto 20 %. It also shows that the Al2O3-water nanofluid performed better than water.