Passive Heat Transfer Enhancement Techniques Research Articles

An Overview of the Recent Advances in Pool Boiling Enhancement Materials, Structrure, and Devices.

This review attempts to provide a comprehensive assessment of recent methodologies, structures, and devices for pool boiling heat transfer enhancement. Several enhancement approaches relating to the underlying fluid route and the capability to eliminate incipient boiling hysteresis, augment the nucleate boiling heat transfer coefficient, and improve the critical heat flux are assessed. Hence, this study addresses the most relevant issues related to active and passive enhancement techniques and compound enhancement schemes. Passive heat transfer enhancement techniques encompass multiscale surface modification of the heating surface, such as modification with nanoparticles, tunnels, grooves, porous coatings, and enhanced nanostructured surfaces. Also, there are already studies on the employment of a wide range of passive enhancement techniques, like displaced enhancement, swirl flow aids, and bi-thermally conductive surfaces. Moreover, the combined usage of two or more enhancement techniques, commonly known as compound enhancement approaches, is also addressed in this survey. Additionally, the present work highlights the existing scarcity of sufficiently large available databases for a given enhancement methodology regarding the influencing factors derived from the implementation of innovative thermal management systems for temperature-sensitive electronic and power devices, for instance, material, morphology, relative positioning and orientation of the boiling surface, and nucleate boiling heat transfer enhancement pattern and scale. Such scarcity means the available findings are not totally accurate and suitable for the design and implementation of new thermal management systems. The analysis of more than 100 studies in this field shows that all such improvement methodologies aim to enhance the nucleate boiling heat transfer parameters of the critical heat flux and nucleate heat transfer coefficient in pool boiling scenarios. Finally, diverse challenges and prospects for further studies are also pointed out, aimed at developing important in-depth knowledge of the underlying enhancement mechanisms of such techniques.

Investigating heat exchanger tube performance: second law efficiency analysis of a novel combination of two heat transfer enhancement techniques

AbstractIn the study, the focus was on evaluating the second law efficiency of a heat exchanger tube operating under continuous heat flux and turbulent flow conditions. The evaluation involved the use of a hybrid GnP and Fe3O4 and modified coiled wire as passive heat transfer enhancement techniques. The primary objective was to investigate the impact of these combined techniques on thermal and hydraulic performance, entropy generation number, Bejan number and second law efficiency. To achieve this, different mass fractions of GnP and Fe3O4 nanoparticles were used in the hybrid nanofluid, along with two forms of modified coiled wire: barrel type and hourglass type. The experimental results indicated that the utilization of hybrid nanofluids and modified helical inserts led to a noticeable improvement in the second law efficiency of the heat exchanger tube. However, it was observed that the differences in entropy generation number and Bejan number between the barrel and hourglass types were not significant, mainly due to higher frictional losses associated with the latter. The highest recorded second law efficiency was 0.416, while the lowest entropy generation number was 0.118. These values were achieved through the combined use of GnP and Fe3O4 with a mass fraction of 0.4% and a barrel-type coiled wire insert with a pitch ratio of 0.5.

EXPERIMENTAL AND NUMERICAL STUDY ON EFFECTS OF NEW-GENERATION FINNED HEAT EXCHANGER ON THERMAL PERFORMANCE OF THERMOELECTRIC COOLING SYSTEMS

In this study, an attempt has been made to increase the efficiency of the thermoelectric refrigerator by designinig a new-generation finned heat exchanger. Surface extension, which is one of the most applied passive heat transfer enhancement techniques, was applied for this finned heat exchanger. In this application, the heat absorbed from the cooling room is transferred to the external environment more effectively. In addition, by using an external thermoelectric element (which is installed with the secondary heat exchanger), the heat exchanger cools down faster and the heat is transferred to the environment more quickly. The manufactured cooling system was tested experimentally under different working conditions, including natural and forced convection. The effects of air velocity and applied voltage to the external TE module on thermal performance were examined. Additionally, the external finned heat exchanger has been simulated and heat transfer characteristics have been evaluated using computational fluid dynamics. The lowest and highest COP values have been obtained as 0.003 and 0.011, respectively, when the external TE module has been passive. By providing 12 V for the external TE module, the lowest and highest COP values have been observed as 0.0031 and 0.0042, respectively. In addition, the importance of surface extension applications for the efficient operation of thermoelectric elements has been emphasized.

Combined passive enhancement techniques improve the thermal performance of latent heat storage system: A design anomaly?

Poor heat transfer rate due to low thermal conductivity of phase change material limits the wide application of latent heat storage (LHS) systems. Thermal performance enhancement techniques can be potential methods to address this major limitation of LHS systems. In this context, the present numerical study examines the individual and cumulative impact of two passive heat transfer enhancement techniques (the orientation of the domain and position of the heat transfer fluid (HTF) tube) on the thermal performance of the latent heat storage (LHS) system consisting of high-temperature (HT) phase change medium (PCM). The individual effect of orientation decreases the charging duration of θ = 0° (horizontal configuration) by 10.5 %, 15 %, and 19.04 % than θ = 30°, θ = 60° and θ = 90° (vertical configuration), respectively. However, the discharging duration remains unchanged for all inclinations (11.5–12 h). The individual effect of eccentricity decreases the charging duration of horizontal (θ = 0⁰, ey = −5 mm,-10 mm,-20 mm) system by 6.25 %,12.8 %, and 18.75 % than concentric horizontal (θ = 0°, ex = ey = 0) system. Moreover, the combined effect of orientation and eccentricity (θ = 0⁰,ey = −5 mm,-10 mm,-20 mm) reduces charging duration by 28.75 %, 33.33 %, and 38.1 % than vertical concentric domain (θ = 90°, ex = ey = 0), respectively. However, eccentricity in vertically upward and downward directions increases the discharging duration compared to the concentric domain. Hence, the combined effect results in a design anomaly for HT-LHS system. The charging and discharging performances are observed to be predominantly sensitive to the Rayleigh number than the Reynolds number.

Experimental and numerical investigations on the effect of a novel internal surface micro-grooving toward improving convective heat transfer performance of tube heat exchangers

In the present work, the use of passive heat transfer enhancement technique through surface alteration was explored. The enhancement was achieved through internal conduit surface micro-grooving using a new apparatus that was developed by modifying a magnetic abrasive finishing technique. A new surface profile was created and later verified using optical and laser profiler measurements. The new profile was numerically investigated to compare the heat transfer and hydrodynamic performance against other profiles that have been studied in the literature. It was found from the results that the new profile shows much higher heat transfer improvement and comparable pressure loss to the previously investigated profiles (i.e., square, rectangular, trapezoidal, and circular). Overall, the new groove geometry provides the highest performance followed by the circular, triangular, curvy, square, and rectangular grooves. Also, the results indicate that designs with a smooth profile performs better than those with sharp edges, owing to the elimination of stationary fluid spots within the grooves. The effectiveness between the profiles was compared based on the level of heat transfer enhancement against the flow penalty. Experimental validation was further conducted for the experimentally generated groove. The results revealed that a relatively small surface temperature drop was obtained, corresponding to a slight improvement in heat transfer. This confirms the results generated by the simulation that groove size plays a major role in attaining significant improvement in heat transfer.

Numerical Investimation of Heat Transfer in A Rectangular Channel with Triangular Vortex Generators in the Channel at Blade Angle 30° for Reynolds no. of 1500, 2000

Abstract: The compact heat exchanger is widely used in fields such as automobile industry, heating and air conditioning, power system, chemical engineering, electronic chip cooling and aerospace, etc. The subject of heat transfer enhancement is of significant interest in developing compact heat exchanger to meet the desire of high efficiency and low cost with the volume as small as possible and the weight as light as possible. The use of ribs/baffles placing in the cooling channels or channel heat exchangers is one of the commonly used passive heat transfer enhancement technique in single-phase internal flows. Periodically positioned ribs/baffles in the channels interrupt hydrodynamic and thermal boundary layers. Downstream of each rib/baffle the flow separates, recalculates, and impinges on the channel wall and these effects are the main reasons for heat transfer enhancement in such channels. the effect of vortex generators in the spanwise averreged nusselt number. The comparison between the plots of spanwise avereged Nusselt number with and without vortex generators shows that there is a considerable increase in Nusselt number near the vortex generator region and this effect decreases as we move far from the vortex generator region in the stream wise direction.

Exploration of ARC-class reactor vessel and divertor cooling system

ARC (Affordable, Robust, and Compact) is a compact tokamak design proposed by Commonwealth Fusion Systems and the Plasma Science and Fusion Center at the Massachusetts Institute of Technology.ARC features REBCO (Rare Earth Barium Copper Oxide) high temperature superconducting magnets, demountable coils, a replaceable vacuum vessel and a liquid immersion blanket (LIB). The LIB is a Li2BeF4 (FLiBe) molten that provides cooling, tritium breeding, and neutron shielding. The compact dimensions of ARC result in very high heat fluxes on the first wall (0.4MW/m2) and the divertor (10MW/m2). These heat fluxes mandate a high heat transfer coefficient in the cooling channels that cannot be achieved without adopting heat transfer enhancement techniques. The scope of this work is to analyze the FLiBe flows through the vacuum vessel channels, and the exploration of different passive heat transfer enhancement techniques to provide adequate cooling to the vessel, which must withstand considerable heat flux and volumetric power generation. Minimum cooling requirements for vessel and divertor are evaluated, and the performance of different turbulence promoters is compared to find the most advantageous design. In particular, wire coil inserts decrease pressure drops by 39% with respect to twisted tapes inserts while providing the same cooling performance.

A Review on Active Heat Transfer Enhancement Techniques within Latent Heat Thermal Energy Storage Systems

Renewable energy resources require energy storage techniques to curb problems with intermittency. One potential solution is the use of phase change materials (PCMs) in latent heat thermal energy storage (LHTES) systems. Despite the high energy storage density of PCMs, their thermal response rate is restricted by low thermal conductivity. The topic of heat transfer enhancement techniques for increasing thermal performance of LHTES systems has mainly focused on passive heat transfer enhancement techniques with less attention towards active methods. Active heat transfer enhancement techniques require external power supplied to the system. In this paper, recent advances in active heat transfer enhancement techniques within LHTES systems are reviewed, including mechanical aids, vibration, jet impingement, injection, and external fields. The pertinent findings related to the field are summarized in relation to the charging and discharging processes of PCMs. Suggestions for future research are proposed, and the importance of additional energy input for storage is discussed.

Augmentation of heat transfer through passive techniques

AbstractThe thermal performance of energy preservation systems is greatly improved by increasing miniaturization and boosting. These are imaginative (or Promethean) techniques to enhance heat transfer. Enhancement methods of heat transfer draw great attention in front of the industrial sector because of their ability to provide energy savings and raise the economic efficiency of thermal systems. Three techniques these methods are categorized; those are active, passive, and compound. Different types of components are used in passive methods because of the transfer/working fluid flow path to the enhancement of the heat transfer rate. In this article, the subject of the review was the passive heat transfer enhancement methods including inserts (conical strips, winglets, twisted tapes, baffles), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), extended surfaces (fins) and nanofluids (mono and hybrid nanofluid). Recent passive heat transfer enhancement techniques are studied in this article as they are cost‐effective and reliable, and also comparably passive methods do not need any extra power to promote the energy conversion systems' thermal efficiency than 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 nanofluid). From the pioneers' research work, it is clear that a lower twist ratio and lower pitch, lesser winglet angles can provide more heat transfer rate and a little bit more friction factor. In the case of nanofluids, a little bit of pumping power is enhanced. Finally, heat transfer enhancement is compared with the thermal performance factor, which is more than unity.

Effect of design parameters on passive control of heat transfer enhancement phenomenon in heat exchangers–A brief review

The development of innovative heat exchangers with improved heat transfer performance is an important concern for heat transfer applications. The use of passive techniques such as vortex generators, surface roughness, augmentation of heat transfer area, or the combination of passive techniques is effective to improve the performance of heat exchangers. This review briefly summarizes the passive heat transfer enhancement techniques and the most important types of heat exchangers. Many effective design parameters have been briefly discussed to improve the performance of passive heat transfer techniques for heat exchangers.

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.

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).

Elliptical Pin Fin Heat Sink: Passive Cooling Control

The aim of this study is to examine by means of three-dimensional numerical simulations the thermal-fluid features in elliptical pin fin heat sink. The passive heat transfer enhancement technique is used to comprehend and control the cooling process. This passive methodology is based on pin fins arrangement, hydrodynamic and geometrical parameters. The present numerical results are confronted with experimental measurements in open literature which used one-dimensional model to explore the thermal field. A good agreement was found especially around the optimal fins dimensions. A parametric study has been carried out to deeply analyse the three-dimensional thermal-fluid fields of the heat sink for various key parameters range such the Reynolds number (Re = 50–250) and the aspect ratio (γ=H/d=5.1-9.18). Some new observations and results are obtained thanks to numerical simulations as tool of investigation. It is shown that the fins circumferential temperature is almost uniform. Furthermore, a better cooling is obtained when the Reynolds number increases mainly when the inlet velocity u0> 0.3m/s. The most suitable value of the aspect ratio is attained for γ=8.16, which ensure an optimal cooling process of the pins. A new global Nusselt number correlation was developed for engineering applications.

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.

Experimental Comparative Analysis of Overall Heat Transfer Coefficient in Counter-Flow Heat Exchanger by using Helical Ribs

More performance or reduced the size of heat exchanger can be achieved by heat transfer enhancement technique. Tube helical ribs have been used as one of the passive heat transfer enhancement technique and are most widely used tube in a several heat transfer process. The results of the heat transfer characteristics in horizontal double pipe with helical ribs are presented. Six test section with different characteristics parameters of helical rib depth 1.0mm, 1.25mm, 1.5mm and helical rib pitch 4mm, 6mm, 8mm, are tested. Cold water and hot water are used as the working fluids in the shell side and tube side respectively. Experiments are performed under the condition of mass flow rate varying from 0.030 to 0.130kg/s for cold water and 0.040 to 0.140kg/s for hot water respectively. The inlet cold and hot water temperature are between 28- 300C and between 68-710C respectively. The results obtained from the tubes with helical ribs are compared with those without helical ribs. It is found that the helical ribs have a significant effect on the heat transfer coefficient and the heat transfer increases with the helical rib pitches and depth. Based on fitting the experimental data, on- isothermal correlations of the heat transfer coefficient and friction factor are proposed.

An integrated optimization methodology for heat transfer enhancement: A case study on nanofluid flow in a pipe equipped with inserts

A hybrid approach that combines Multi-objective Optimization (MOO) and Multiple-criteria Decision-making (MCDM) techniques has been introduced in the present paper for heat transfer augmentation in pipes. The combination of two passive heat transfer enhancement techniques namely the nanofluid and insert employment have been applied in the representative problem. In this context, the performance of water-based CuO,Fe3O4, and CNT-Fe3O4 nanofluids and two different inserts as twisted tape (TT) and wire coil (WC) were evaluated and their optimum design and flow parameters were determined. The methodology introduced here has two major parts. Firstly, a MOO study was conducted employing the experimentally obtained correlations for the Nusselt number (Nu) and the friction factor (f) as objective functions. The optimum values of the selected design parameters (the Reynolds number (Re), nanoparticle volume fraction ((ϕ)), twist ratio (h/d), wire coil pitch ratio (p/d), and the Prandtl number (Pr)) that maximizes Nu while minimizing f were found as Pareto front. Secondly, the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method was used to help decision-makers making an optimal choice among the Pareto optimal solutions that meet their requirements. In the MCDM problems, two criteria (Nu and f) and fifteen alternatives were evaluated for each case. A sensitivity analysis was also conducted by changing the weights of criteria. It was found that the combination of TT and CNT-Fe3O4 with the values of design parameters (Re; ϕ; h/d; Pr)=(20,000; 0.27%; 10; 5.50) exhibited the best thermal-hydraulic performance for the equal weights of criteria.

Performance evaluation of helical coils as a passive heat transfer enhancement technique under flow condensation by use of entropy generation analysis

Current research focuses on the performance of helically coiled tubes as a passive heat transfer enhancement technique for R134a flow condensation from an entropy generation perspective. Similar to other enhancement techniques, helical coils, are accompanied by pressure drop as a penalty, diminishing their performance, so that these coils are of lower performance compared to straight tubes where the increase in entropy generation due to pressure drop overcomes the decrease in entropy generation due to enhanced heat transfer. Unlike previous studies that have largely investigated heat transfer and pressure drop characteristics of helical coils regardless of their performance, this study employs entropy generation analysis as an effective method to distinguish flow and geometrical conditions at which helical coils are of lower entropy, i.e. higher performance, compared to straight tubes. The findings reveal that, for both helical and straight tubes, entropy generation is enhanced as tube diameter, mass velocity, vapor quality, and wall heat flux increase and saturation temperature decreases. Additionally, applying helical coils within wider ranges of mass velocities can be justified at lower values of tube (≤8.3 mm) and coil diameters (≤200 mm), saturation temperatures (≤40 °C), and vapor quality (≤0.6), and at higher values of wall heat flux (≥15 kW/m2). These results substantiate that employing helical coils in lieu of straight tubes is not justifiable always (for all flow and geometrical conditions) although they are of superior heat transfer performance compared to straight tubes.

Investigation of storage rotation effect on phase change material charging process in latent heat thermal energy storage system

Applying Phase Change Materials (PCMs) in Thermal Energy Storage (TES) systems is an appropriate method to utilize renewable energies, more efficiently. Due to the low thermal conductivity of the PCMs, the application of active and passive heat transfer enhancement techniques is increasing. In this study, a double-pipe Latent Heat Thermal Energy Storage (LHTES) System in which the annulus is filled with PCM is considered and storage rotation method is suggested as an active heat augmentation technique to accelerate the charging process of the systems. Different scenarios are considered for rotations, including constant speed and step-by-step rotation methods. The charging process of a LHTES system is simulated numerically considering 2-D model, and the effect of different scenarios of the storage rotation is investigated on charging time. The results show that rotating the storage with constant rotational speed has a slight positive effect on complete melting process time (maximum reduction of 3.45%), however 15.5% reduction in charging time can be achieved compared to the fixed mode by applying step-by-step rotation scenario which is related to the case with two opposite direction rotations and 180 angle difference between the stop steps.

Experimental Study of Heat Transfer Enhancement by Turbulence Generator in Side Pipe

The enhancement of heat transfer exchangers is an important matter of concern for achieving more efficient thermal energy conversion systems. The use of modified twisted tape (TT) inserts as a passive heat transfer augmentation technique is an efficient way of improving heat transfer. Twisted tape insert is one of the passive heat transfer enhancement techniques that are widely used in various applications for heat transfer, such as air conditioning and cooling systems, heat recovery systems, Processes, procedures for food and milk, plants for chemical processes. To increase the heat transfer rate of the tubular heat exchanger, in this study a small-scale experimental setup was performed. The heat improvement of the heat exchanger is done using twisted tape inserts with a twist ratio (y=5.5). The effects of two-dimensional parameters namely Nusselt number & friction factor on tubular heat exchanger effectiveness are studied. The turbulent flow was generated by the insertion into the test pipe of the twisted tape inserts, producing a high turbulence rate in the pipe, resulting in an increase in turbulence. And hence the enhancement of heat transfer and drop in pressure. The length and width of the insert were 1000 mm and 21.5 mm respectively. The diameter of the test pipe outside & inside is 23 mm and 22 mm respectively. The length of the test section is 2000 mm. The study is carried out for the Reynold number regime (500-6500). The results show that The maximum enhancement for Nu number at Re 500 was 32% that of the plain tube.

An experimental investigation on bubbles departure characteristics during sub-cooled flow boiling in a vertical U-shaped channel utilizing high-speed photography

Classified as a passive heat transfer enhancement technique, U-shaped channels are commonly encountered with applications associated with boiling heat transfer such as air conditioning systems, evaporators, and boilers. On the other hand, understanding heat transfer in flow boiling is heavily dependent upon bubbles dynamic behavior, which is influenced by flow and geometrical conditions. In this paper, bubbles departure characteristics are experimentally studied in upward sub-cooled flow boiling of distilled water in a vertical U-shaped channel. The influence of flow conditions, embracing heat flux, mass flux, and inlet sub-cooling on bubbles characteristics, encompassing departure diameter, growth and waiting times, and nucleation frequency, is investigated through conducting 68 experiments. All the experiments are carried out at atmospheric pressure over a Nichrome heating surface installed on the outer wall of the channel, with heat flux, mass flux, and inlet flow temperature in the ranges of 26.1–61.5 kW.m-2, 114–255kg.m-2s-1, and 1 to 8 °C. A high-speed camera is utilized to capture bubbles growth process and departure instance. The results reveal that as wall heat flux increases and mass flux and inlet sub-cooling decrease, bubbles departure diameter and frequency rise, while waiting time declines. Also, lower growth times are detected when wall heat flux and mass flux increase and inlet sub-cooling declines. Two new correlations are developed to predict present and previously published experimental data of departure diameter and nucleation frequency with mean standard deviations of 18.0% and 18.5%, correspondingly.