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Pulsating heat pipes are widely used in cooling electronic devices, automotive radiators, refrigeration, and air conditioning systems. The addition of nanoparticles is one of the effective ways to improve the heat transfer characteristics of fluids. This paper, based on large eddy simulation (LES) numerical method and field synergy theory, investigates the heat transfer characteristics of Al2O3/Cu mixed nanofluid in a self-excited oscillating heat exchanger tube. Through multi-objective optimization of field synergy angles, field synergy factors, and structural parameters, the enhanced heat transfer mechanism of mixed nanofluids in a self-excited oscillating heat pipe is investigated. It is found that there is a significant negative correlation between the heat transfer coefficient h at the wall surface of the heat exchanger tube and the synergistic angle β. The incorporation of nanoparticles improves the synergistic properties between the velocity and temperature fields. The optimum structural parameters are L/D = 0.469931, α = 128.9904° and d2/d1 = 2.047937, and the optimization increases Nu by 1.92%, decreases f by 10.46%, and decreases the temperature synergy angle β by 2.24°. At the same time, the orthogonality or lack of orthogonality between isotherms and streamlines determines the extent of synergistic performance in the countercurrent vortex. [ABSTRACT FROM AUTHOR]

This paper examines experimentally the effect of jet vortex technology on enhancing the heat transfer rate within a double pipe heat exchanger by supplying the heat exchanger with water at different vortex strengths. A vortex generator with special inclined holes with different inlet angles was designed, manufactured, and integrated within the heat exchanger. In this study, four levels of Reynolds number for hot water in the annulus (Reh) were used, namely, 10,000; 14,500; 18,030; and 19,600. Similarly, four levels of Reynolds number for cold water in the inner tube (Rec) were used, namely, 12,000; 17,500; 22,500; and 29,000. As for the inlet flow angle (θ), four different levels were selected, namely, 0°, 30°, 45°, and 60°. The temperature along the heat exchanger was measured utilizing 34 thermocouples installed along the heat exchanger. It was found that increasing the inlet flow angle (θ) and/or the Reynolds number results in an increase in the local Nusselt number, the overall heat transfer coefficient, and the ratio of friction factor. It is revealed that the percentage increase in the average Nusselt number due to swirl flow compared to axial flow was 10%, 40%, and 82% for an inlet flow angle of 30°, 45°, and 60°, respectively. [ABSTRACT FROM AUTHOR]

A recent study compared experimentally the hydraulic and thermal activity of twisted tape inserts for two types, metal foam twisted tape (MFTT) and traditional twisted tape (TTT), in a double pipe heat exchanger. The investigation goal of the innovatively designed MFTT is to enhance the heat transfer process, which provides a higher thermal enhancement factor over those of TTT under the same conditions. Heat transfer activity in terms of Nusselt number (Nu) and Nusselt number ratio (Nu/Nuo) as the heat exchanger equipped with the proposed MFTT and TTT inserts with various tape‐twist ratios (y/W = 3.6, 5.2, and 6.8) are considered over various values of Reynolds numbers (16,300 < Re < 32,500). The results reveal an improvement in heat transfer rate with both MFTT and TTT and this improvement is proportional inversely to the twisted ratio, but the MFTT showed the highest value of heat transfer compared to TTT and clear tube. Also, the Nu of MFTT augments averaged about 175%–230% compared to the TTT depending on the twist ratio and about 275% and 500% over that of the clear tube. At a twist ratio of 3.6, the heat transfer coefficient is recorded at 37% and 45% for MFTT and TTT models, respectively, with an increase in Re. However, friction factor (f) increases about 35% for MFTT with decreasing in twist ratio (from 6.8 to 3.6). The effectiveness of the proposed MFTT in the range of Re used is found to be about 12%, 15%, and 18% higher than those induced by TTT at twisted tape ratio (y/w = 3.6, 5.2, and 6.8), respectively. [ABSTRACT FROM AUTHOR]

In this paper, the effects of integrated fins on the thermal performance of the concentric tube heat exchanger with a variable flow rate of hot and cold water are discussed. The inner pipe is made of copper pipe of length = 800 mm, OD=30 mm, ID=26 mm, t=2 mm. the integrated fines of the geometry 1.25 mm and 1.5 mm pitch length, 1 mm of the depth of cut are considered in the study. The thermal performance of the heat exchanger with a flow rate of hot and cold fluids ranges from 0.014 kg s-1 to 0.07 kg s-1 and the effects of parallel and counter flow direction are also discussed in detail. The experimental result confirms that the thermal performance of the concentric tube heat exchanger is more in the case of the tube with integrated fins and counter flow direction with the highest flow rate of water in the pipe and annulus of the heat exchanger. [ABSTRACT FROM AUTHOR]

This paper considers the main parameters of the solar collector with thermosyphon circulation for the solar heat supply system. There was developed a flat solar collector construction with thermosyphon circulation wherein the heat transfer coefficient is increased by removing the additional partitions between a panel and heat insulation. The solar collector's efficiency reached by the availability of a tank and thermal pump in the construction where a condenser and evaporator executed the form of a heat exchanger of «spiral in spiral» type and heat exchanger pipe connections are one above another, which allows increasing the square and heat exchange intensity. The outcome of the study is a description of the mathematical analysis of the heat transmission process in the tank accumulator. We have shown a heat carrier thermophysical parameter dependence on the temperature. [ABSTRACT FROM AUTHOR]

This paper discusses a prototype of the ORC, which is developed utilizing a novel multi-coil evaporator and condenser for the waste heat source with a temperature below 100 °C. R123 acts as a working fluid for the ORC. The thermal performance of a multi-helical coil is evaluated experimentally and compared to the single-helical coil heat exchanger available in the literature. The performance of a novel multi-coil heat exchanger depicts that when the Reynolds Number varies from 60000 to 120000, the heat transfer coefficient varies from 1560 W/(m2K) to 2900 W/(m2K) for a single coil heat exchanger and 3014 W/(m2K) to 4531 W/(m2K) for multi helical coil evaporator. The Nusselt Number varies from 300 to 570 for single-coil heat exchangers and 715 to 1076 for multi-helical coil heat exchangers. The results show that the expander's power output is around 2.14 kW and proportional to the evaporator temperature and pressure increase. The high condensing temperature decreases the expander power output. The main contribution of the present work is developing a small ORC system with novel multi-helical coil heat exchangers and testing them on a lab scale for the most probable application to fulfill the electricity requirement in remote areas in the future. [Display omitted] • The novel multi-helical coil heat exchanger (MHCHE) designed, manufactured, and tested. • Novel MHCHE utilized in small-scale organic Rankine cycle's lad scale testing. • Impact of experimental parameters on power output and thermal efficiency of ORC-MHCHE assessed. • Usefulness of a small-scale ORC-MHCHE demonstrated at low temperature (<100 °C). • R123 and water acts as working and heat transfer fluid in a small-scale ORC-MHCHE. [ABSTRACT FROM AUTHOR]

The paper presents numerical investigations of a three fluid heat exchanger (TFHE), which is an improvement on the double pipe heat exchanger, where a helical tube is inserted in the annular space between two straight pipes. The helical tube side fluid, that is, hot water continuously transfers heat to the outer annulus side fluid and innermost tube side fluid. The heat transfer and pressure drop characteristics of the TFHE are assessed for different flow rates and inlet temperatures. With an increment in the volumetric flow rate of the helical tube side fluid and outer annulus side fluid, the overall heat transfer coefficient increases, and the effectiveness decreases for heat transfer from the helical tube side fluid to outer annulus side fluid in both parallel flow and counter flow configurations. It is also observed that with increment in the helical tube side fluid inlet temperature, the overall heat transfer coefficient and effectiveness increases for heat transfer from the helical tube side fluid to outer annulus side fluid in both flow configurations. The parameter, JF factor, has been proposed to evaluate the thermohydraulic behavior of the TFHE, where it is obtained that the behavior of the TFHE is better at a lower helical tube side fluid velocity and higher outer annulus side fluid velocity. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer analysis for an air-air heat exchanger was experimentally carried out to find its thermal performance and effectiveness. Air-air heat exchanger equipped with finned heat pipes was considered for the experimentation. Mass flow rate of air was considered in between 0.24 to 0.53 kg/s. The temperature at the condenser side of the heat pipe heat exchanger was kept constant at around 23 °C and at the evaporator part it was varied from 88 to 147 °C. The performance of heat pipe heat exchanger was evaluated at different mass flow rate of air, in terms of effectiveness and compared with its corresponding value found by theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper proposes a methodology aiming at determining the most influent working variables and geometrical parameters over the pressure drop and heat transfer during the condensation process of several refrigerant gases using heat exchangers with pipes mini channels technology. A multi-criteria decision making (MCDM) methodology was used; this MCDM includes a mathematical method called SIMUS (Sequential Interactive Modelling for Urban Systems) that was applied to the results of 2543 tests obtained by using a designed refrigeration rig in which five different refrigerants (R32, R134a, R290, R410A and R1234yf) and two different tube geometries were tested. This methodology allows us to reduce the computational cost compared to the use of neural networks or other model development systems. This research shows six variables out of 39 that better define simultaneously the minimum pressure drop, as well as the maximum heat transfer, saturation pressure fluid entering the condenser being the most important one. Another aim of this research was to highlight a new methodology based on operation research for their application to improve the heat transfer energy efficiency and reduce the CO2 footprint derived of the use of heat exchangers with minichannels. [ABSTRACT FROM AUTHOR]

• Critical review of morphological and thermal-hydraulic performance of the conventionally and additively manufactured lattices is conducted. • Conventionally manufactured wire-meshes can provide heat transfer enhancements similar to stochastic metal foams and relatively easy and cheap to manufacture. • Additively manufactured lattices demonstrate higher interfacial heat transfer coefficient values than stochastic metal foams, but the role of inherent surface roughness needs more investigation. The advantages offered by the stochastic metal foams when used in high-performance compact heat exchangers is evident in the open literature. Recently, there has been a surge in investigations on regular metal lattices which can be manufactured additively, because of their "potential" to provide multifunctional and tunable characteristics to heat exchangers. These lattices are also reported to lend superior mechanical properties as well. The literature, however, lacks in the critical assessment of these additively manufactured regular lattices in terms of the quality of printed parts (defects, surface roughness, etc.), their compatibility in compact heat exchangers, flow and thermal transport characteristics, and whether or not they are viable alternatives to the commercial metal foams conventionally manufactured. This paper reports different manufacturing routes for the fabrication of conventional stochastic and regular lattices and additively manufactured regular lattices, and how these technologies have evolved over the years. The additively manufactured lattices different from the commercial TKD may result in significantly higher interstitial heat transfer coefficient. Further, the design freedom allows different porosities through which higher effective thermal conductivity can be achieved. The usage of regular lattices in tube-heat exchangers, as wicks in heat pipes, heat transfer enhancement features in boiling heat transfer and transpiration cooling mechanisms of turbine blades as an alternative to conventional technologies is promising. The role of inherent surface roughness in dictating the thermal and flow transport in additively manufactured lattices, especially for multi-phase flow applications, should be studied in more detail. [ABSTRACT FROM AUTHOR]

In this paper, the performance of a multi-pass heat pipe-based heat exchanger (HPHE) is investigated experimentally and theoretically. The heat pipe system consists of copper heat pipes in a specific equatorially staggered configuration to facilitate heat transportation from a hot gas (air) to a water flow, which cools the condenser section of these heat pipes. The effect of the Reynolds number on the heat transfer rate was studied by altering the number of passes for the evaporator section for the same system by the incorporation of various baffles and by varying the water flow rate. The experimental results have highlighted the strong correlation between heat exchanger performance and the Reynolds number. By increasing the number of passes from one to five, the effectiveness of the HPHE was improved by more than 25%. It has been demonstrated that increasing the number of passes increases the Reynolds number of the flow, leading to higher heat transfer coefficients and lower thermal forced convection resistances. The HPHE overall performance, as well as, the outlet temperatures of the fluids were predicted through two theoretical models, based on the Log Mean Temperature Difference (LMTD) method and the Effectiveness-Number of Transfer Units (ε-NTU) method. The predictions were compared with experimental results and the accuracy of the models reported. The validation showed that the developed iterative LMTD model predicted the performance of the HPHE within ±15.5% error. In comparison, the ε-NTU model predicted the total effectiveness with a maximum error of 19% and was able to predict the outlet temperatures of both air and water streams within an accuracy of ±0.7 °C. The reported research is of importance for the application of heat pipe heat exchangers in waste heat recovery. Finally, knowledge is provided on the accuracy of the available prediction models. [ABSTRACT FROM AUTHOR]