Excellent Heat Transfer Performance Research Articles

Cryogenic Oscillating Heat Pipe for Conduction-Cooled Superconducting Magnets

The cryogenic oscillating heat pipe (OHP) is considered as a convenient thermal link between the crycooler head and the superconducting magnet due to the employment of flexible capillary tube. In this paper, a cryogenic OHP using nitrogen as the working fluid is fabricated. The copper blocks connecting with the evaporator of the cryogenic OHP are used as a mock-up superconducting magnet, and a heating sheet is attached on the copper blocks to simulate the heat generation of the superconducting magnet system during the operation process. The cooling down processes and the operation characteristics of the cryogenic OHP at the different condenser temperature are investigated. The results show that the cryogenic OHP has not only large cooling capacity during the cooling down process, but also excellent heat transfer performance during the operation process.

Numerical Study of Compact Plate-Fin Heat Exchanger for Rotary-Vane Gas Refrigeration Machine

Plate-fin heat exchangers are widely used in refrigeration technique. They are popular because of their compactness and excellent heat transfer performance. Here we present a numerical model for the development, research and optimization of a plate-fin heat exchanger for a rotary-vane gas refrigeration machine. The method of analysis by graphic method of plate - fin heat exchanger is proposed. The model describes the effects of secondary parameters such as axial thermal conductivity through a metal matrix of the heat exchanger. The influence of geometric parameters and heat transfer coefficient is studied. Graphs of dependences of length, efficiency of a fin and pressure drop in a heat exchanger on the thickness of the fin and the number of fins per meter are obtained. To analyze the results of numerical simulation, the heat exchanger was designed in the Aspen HYSYS program. The simulation results show that the total deviation from the proposed numerical model is not more than 15%.

Experimental Investigation on Solid Particle Flow Characteristics in Particle Curtain Heat Exchanger

The particle curtain heat exchange, which is based on the rapid heat equilibrium theory of gas-solid two-phases, has attracted more and more attentions of researchers due to its flexible system arrangement, excellent heat transfer performance and real-time adjustable heating surface etc. In order to provide data for optimal design and operational control of the curtain exchanger, experiments were conducted to investigate flow characteristics of sand particles and geometry of the curtain under different conditions such as air velocity at the inlet, initial thickness of particle curtain, diameter of sand particle and mass flow rate of particles etc.

Investigation of thermo-fluidic performance of phase change material slurry and energy transport characteristics

Thermal or cold storage is a promising way to alleviate the peak-valley difference of the electricity load and improve the energy efficiency. The thermal or cold storage by micro-encapsulated phase change material (MPCM) slurry is one of the effective measures to be implemented in waste heat recovery and heat transport system etc. due to its high energy storage density and excellent heat transfer performance. The thermo-fluidic performances of the MPCM slurry flowing through the horizontally circular pipe under constant heat flux are numerically investigated based on the Eulerian-Eulerian approach in the present study. It is found that the numerical results are in good consistence with the experimental results from the aspects of flow and heat transfer. The influences of particle diameter on solid volume fraction distribution, solid velocity distribution and pressure drop of the MPCM slurry are investigated under isothermal condition. And then the temperature distribution of the MPCM slurry and liquid volume fraction of the PCM in MPCM are presented to analyze the heat transfer performance of the MPCM slurry. The particle diameter also imposes significant influences on the heat transfer between the two phases and average heat transfer coefficient of the MPCM slurry. The largest pressure drop and the highest average heat transfer coefficient appear when the particle diameter of the MPCM decreases to 1μm. Further investigations at different Reynolds numbers are carried out to study the variation of local heat transfer coefficient along the pipe. Finally, the comparison of energy transport performances between the MPCM slurry and water are presented based on the ratio of transported heat to pumping power. The MPCM slurry shows better energy transport performance than pure water when the Reynolds number is above 7865.

A high efficient semi-open system for fresh water production from atmosphere

A high efficient semi-open system of fresh water production is established with novel consolidated composite sorbent. This device collects 14.7 kg of water with packing 40.8 kg of consolidating sorbents in 0.4 × 0.4 × 0.6 m size of sorbent bed. The consolidated active carbon felt combined with LiCl sorbent and its corrugated filling mode are invented, which has large cycle sorption quantity, excellent heat transfer performance, and enough mass transfer channels. The sorption and desorption performances of device are tested at different experiments conditions. In sorption process, 14.7 kg, 13.6 kg, and 12.5 kg fresh water is obtained under the condition of 85%, 75%, and 65% RH; while in desorption process, 14.5 kg, 13.6 kg and 0 kg water is got under the condition of 90 °C, 77 °C, and 60 °C respectively. This appliance ensures the large adsorbing capacity at 23 °C and 90% RH, and achieves a large amount of desorption (0.65 g/g) between 70 and 80 °C with 8.8 Pa flow resistance. The pressure drop and velocity distribution of the actual operation in the unit structure of sorbent bed are simulated, and the water mass is calculated to analyze the sorption and desorption performances of the device.

Numerical and experimental research on the grinding temperature of minimum quantity lubrication cooling of different workpiece materials using vegetable oil-based nanofluids

This study investigated the grinding temperature of minimum quantity lubricant cooling (MQLC) for heat transfer. Three typical workpiece materials, namely, 45 steel, Ni-based alloy, and nodular cast iron, were surface grinded. These materials are the most frequently used for mechanical processing. Palm oil with good lubrication and heat transfer performance was chosen as the base oil for the nanofluids. Carbon nanotube (CNT) nanofluids with volume fractions of 2 and 2.5%, as well as excellent heat transfer performance, were prepared for the MQLC fluid. Results showed that the 45 steel grinding had the highest temperature (363.9 °C), and the grinding temperature of the 2% nanofluid (363.9 °C) was slightly higher than that of the 2.5% nanofluid (352.9 °C). A numerical simulation heat transfer model conducting the finite difference method was established for the numerical analysis of the MQLC grinding temperature. Results indicated that the model predictions and experimental results are in good agreement, with 4.8% average model error. The heat transfer mechanism of the nanofluids was also analyzed. This study confirmed that nucleate boiling heat transfer was achieved when grinding the Ni-based alloy.

Phase change materials development from salt hydrate for application as secondary refrigerant in air-conditioning systems

Salt hydrate for application as secondary refrigerant in the air-conditioning system are selected based on the slurry forming according to the evaporator temperature 5°C–12°C. Research development of salt hydrate for application is done in three stages, namely; the study of the properties, flow, and heat transfer characteristics, also applications in air-conditioning systems. The study of the properties of the phase change materials is in the form of latent heat, freezing and melting temperature, thermal stability properties, viscosity, and rate of corrosion. The study of flow and heat transfer characteristics determine the effect of salt hydrates on the pressure drop and heat transfer in a heat exchanger. Studies on the application of the air-conditioning system is made to use a type of cooling room air-handling units and fan coil unit. The results of the researches for each of two stages are described in the discussion of the current article. The characteristics of salt hydrates from Na2HPO4 and CaCl2 are fitted to be applied as secondary refrigerant. This material has high latent heat value and matched phase change temperature, as well as a very low corrosion rate. This material also demonstrates excellent heat transfer performance, in which an increase as much as 18.62% for salt hydrate from calcium chloride and 13.9% for salt hydrate from disodium hydrogen phosphate. For its flow characteristics, there was a less significant increase on pressure drop for Na2HPO4.

Preparation of flexible carbon nanotube ropes for low-voltage heat generator

Carbon nanotube (CNT) ropes (∼1.5 mm in diameter and ∼3000 mm in length) possessing a high degree of flexibility were prepared by twisting CNT socks with the use of rotating motors. Such ropes were not only highly flexible but also dense and homogenous. The structure also allowed the ropes to be easily manipulated without any damage. When the ropes were employed as a heat generator, white incandescence on the rope could be clearly observed even at a low voltage. The properties of the ropes were also validated by observing that water could be boiled after heating for several minutes at a voltage of 15 V. Furthermore, the content of elements in the water before and after heating was unchanged, which demonstrated the structural stability of the ropes. The results indicated the excellent heat transfer performance of the flexible ropes and exhibited potential for use in portable and safe heat generator technology.

Comprehensive performance comparison of airfoil fin PCHEs with NACA 00XX series airfoil

Printed circuit heat exchanger (PCHE) can be used in supercritical carbon dioxide (S-CO2) Brayton cycle. The present study compares NACA 0020 airfoil fin PCHE with conventional zigzag PCHE by numerical analysis. Pressure drop of the former has a striking reduction while maintaining excellent heat transfer performance. Comparison on four NACA 00XX series airfoil fin PCHEs is performed to investigate the influence of airfoil profile on flow and heat transfer performance. With a fixed vertical pitch, heat transfer performance of NACA series airfoil fin PCHE increases as airfoil thickness increases. However, comprehensive performance, in which both flow and heat transfer are taken into account, degrades with increasing airfoil thickness. Among four NACA airfoil fin PCHEs presented in this paper, NACA 0010 airfoil fin PCHE demonstrates the best comprehensive performance.

Boundary layer redevelops with mesh cylinder inserts for heat transfer enhancement

In this paper, mesh cylinder inserts were adopted to enhance the convective heat transfer in a circular tube. The effect of open area rate (P=0.1mm, P=0.2mm, P=0.4mm), inlet closed type, spacer length (S/D=50/12, S/D=75/12, S/D=100/12) and insert length (L/D=25/12, L/D=50/12, L/D=100/12) on heat transfer performance were numerically investigated at the Reynolds number of 280–1800. The results show that mesh cylinder inserts has an excellent heat transfer performance in the laminar regime. It was mainly due to the high Nu number produced by boundary layer redevelopment at the inlet and the complex flow at the annular region also contributes to the high heat transfer rate. The excellent heat transfer performance is obtained at the cost of high flow resistance. To avoid sharply increase in the flow resistance, a small hole is cut in the closed inlet of mesh cylinder inserts, which can greatly decrease the friction factor without sharply decrease the heat transfer rate. The open area rate has little impact on the heat transfer rate and the best heat transfer performance is resulted from inserts with P=0.2mm. Heat transfer performance of mesh cylinder inserts is also affected by the spacer length, insert length. A relatively long spacer length (S/D=100/12) and a short insert length (L/D=25/12) are recommended in the design of heat exchanger in this work.

Numerical study of the laminar flow and heat transfer characteristics in a tube inserting a twisted tape having parallelogram winglet vortex generators

The usage of wings or/and winglets vortex generators (VGs) could provide excellent heat transfer performance with a minimal increase in pressure loss comparing with other VGs in a wide range of Reynolds number. For classical twisted tape, the contact area between twisted tape and working fluid is large, which would result in high friction drag. Motivated by this fact, a newly designed twisted tape having parallelogram winglet VGs (PWVGs) is proposed to enhance heat transfer. This twisted tape with winglet VGs is formed by cutting at the edge of the straight tape with parallelogram shape to produce a parallelogram winglet as twisting around its longitudinal axis. The laminar fluid flow and conjugate heat transfer characteristics in a circular tube inserting the twisted tape with PWVGs were analyzed using numerical method under uniform wall temperature condition, and four different attack angle α (α=27.64°, 21.44°, 17.44° and 14.67°) and four different axial spacing St (St=0.83D, 1.0D, 1.25D and 1.67D). The results show that (1) the newly designed twisted tape has two ways to generate secondary flow, including secondary flow generated by the twisted base tape and extra secondary flow generated by PWVGs; (2) the combination of twisted base tape and PWVGs has evident effects on the local flow and heat transfer characteristics; (3) compared with the reference empty tube, the twisted tape with PWVGs can effectively enhance heat transfer under identical pumping power condition, for studied case of α=17.44°, St/D=1.25 and Re ranging from 50 to 600, the average Nusselt number and friction factor are increased by 76.4–190.9% and 179.9–289.1%, respectively, and the corresponding thermal performance factor ranges from 1.25 to 1.85; and (4) as Re ranges from 50 to 600, the helical angle α of PWVGs is a very important parameter in design of the modified twisted tape, and when α is large and St is small the heat transfer performance of the twisted tape with PWVGs is better.

Viscous warm pressure bulging process of AZ31B magnesium alloy with different ellipticity dies

Based on the bulging principle of different ellipticity dies, the methyl vinyl silicone rubber with excellent thermal stability and heat transfer performance was chosen as the viscous medium. The finite element analysis and experiments of viscous warm pressure bulging (VWPB) of AZ31B magnesium alloy were conducted to analyze the influence of different ellipticity dies on the formability of AZ31B magnesium alloy. At the same time, based on the grid strain rule, the forming limit diagram (FLD) of VWPB of AZ31B magnesium alloy was obtained through measuring the strain of bulging specimens. The results showed that at the temperature range of viscous medium thermal stability, the viscous medium can fit the geometry variation of sheet and generate non-uniform pressure field, and as the die ellipticity increases, the difference value of non-uniform pressure reduces. Meanwhile, according to the FLD, the relationship between part complexity and ultimate deformation was investigated.

Experimental study on the performance of capillary pump loop with different operation conditions

The capillary pump loop (CPL) has a strong capacity of heat dissipation. But it is interfered by many external factors when it works. In order to study the operating characteristics of CPL in different complex conditions, this paper varies the working fluid, initial vacuum, evaporator inclination, the relative height between the evaporator and condenser, and the relative position between the two. Stability test is carried out in the end. Through the experiment, it can be concluded that ethanol is selected as working fluid; the CPL achieves better performance when the initial vacuum was less than 1,000 Pa; the evaporator inclination and relative height have little impact on the heat transfer performance of CPL; beyond the anti-gravity situation, the CPL exhibits excellent heat transfer performance with the variation of the relative position. The CPL exhibits a decent stability and reliability performance after long time operating.

Experimental study on the performance of capillary pump loop with different operation conditions

The capillary pump loop (CPL) has a strong capacity of heat dissipation. But it is interfered by many external factors when it works. In order to study the operating characteristics of CPL in different complex conditions, this paper varies the working fluid, initial vacuum, evaporator inclination, the relative height between the evaporator and condenser, and the relative position between the two. Stability test is carried out in the end. Through the experiment, it can be concluded that ethanol is selected as working fluid; the CPL achieves better performance when the initial vacuum was less than 1,000 Pa; the evaporator inclination and relative height have little impact on the heat transfer performance of CPL; beyond the anti-gravity situation, the CPL exhibits excellent heat transfer performance with the variation of the relative position. The CPL exhibits a decent stability and reliability performance after long time operating.

Research on the Heat Transfer and Flow Characteristics of a New Type of Aluminum Noncontact Thermal Resistance Finned Tubes

With energy consumption increasing sharply and copper resource decreasing rapidly, the demand for a high-performance and efficient heat exchanger becomes more and more pressing. Therefore, it is very important to develop new technology, new materials, and low-cost and high-performance fin tubes, which is the main component of heat exchanger. In this paper, a new type of aluminum noncontact thermal resistance finned tube (ANCRF) is studied experimentally on the standard wind tunnel test bench. The performance curves of heat transfer and resistance and the experimental correlations were obtained in the experiment range of Reynolds number, which indicates that ANCRF has excellent heat transfer and flow resistance performance. Compared with a louvered fin tube and plate fin tube, the air-side convective heat transfer coefficient of ANCRF was approximately 63.06 and 176.33% higher on average; the friction factor was approximately −71.28 and 51.3% higher on average; and the heat transfer factor was approximately 135.3 and 105.9% higher on average. In addition, a simulation model was established. The positive correlation between the simulated results and the experimental data demonstrated the effectiveness of the model. With this model, the heat transfer process of ANCRF was simulated, with results indicating that the structure sizes of fin thickness, fin height, fin spacing, and fin angle all have great influence on the heat transfer and flow performance of ANCRF, and the rules of influence are different. This research has a guiding significance for the optimization design and application extending for this new type of fin tube.

Thermal characteristic of a laminar flow through a circular tube fitted with wire-net cylinder inserts

A novel type of porous media inserts, wire-net cylinder inserts, was developed to improve convection heat transfer in the present work. Heat transfer performance and pressure drop was numerically studied. Wire-net cylinder inserts with different spacer length (S/D=2.1, S/D=4.2, S/D=8.3) and different open area rate (OAR=0.33, 0.5 and 0.67) were investigated at the Reynolds number range of 290–2024. Liquid water was used as the working fluid. The results show that wire-net cylinder inserts have an excellent heat transfer performance in the laminar regime. This was mainly due to the increase in the velocity at the inlet and the high intensity of the fluid mixing between annular region and the core region. The open area rate has a great impact on the heat transfer rate. The best heat transfer rate resulting from inserts with P=0.2mm (OAR=0.5) is about 3.45 times that of smooth tube. The friction factor increases with the decrease in open area rate. Although inserts with P=0.4mm (OAR=0.67) have the lowest heat transfer rate, its friction factor is also the lowest. As a result, inserts with P=0.4mm achieve the best PEC values (performance evaluation criterion). Heat transfer performance of wire-net cylinder inserts is also affected by the spacer length. The heat transfer rate increases with the decrease in the spacer length. Although a short spacer length can result in a high friction factor, the PEC values are still very high. It is still valuable to adopt a short spacer length in the design of heat exchanger. Compared with other inserts, wire-net cylinder inserts are simple and can be fabricated easily at a low cost.

A numerical study on thermal-hydraulic characteristics of turbulent flow through a circular tube fitted with pipe inserts

A numerical investigation is carried out for turbulent flow in a circular tube with pipe inserts. Pipe inserts with different node installation are carefully investigated at the Reynolds number range of 2892–28,915. Liquid water is used as the working fluid. The results show that the pipe inserts have an excellent heat transfer performance in the turbulent regime. The heat transfer rate decreases with the increase in spacer length. However, the flow resistance increases with the decrease in spacer length. Therefore, a suitable spacer length has to be taken into consideration in the design of heat exchanger. Compared with three pipe inserts, four pipe inserts have a better heat transfer performance, especially at high Reynolds number. The maximal PEC (performance evaluation criterion) values resulting from four pipe inserts were approximately 1.4–3.0. Heat transfer performance of pipe inserts is also affected by the installation angle between insert nodes. Although installation angle has a limited impact on the heat transfer rate, the friction factor increase is also very low. It is still valuable to adopt an installation angle for heat transfer enhancement.

Toward improved engine performance with crumpled nitrogen-doped graphene based water–ethylene glycol coolant

Crumpled nitrogen-doped graphene nanosheet (CNDG) is considered a promising carbon-based nanosheet due to its high surface area, superior electrical, thermal and mechanical properties. Herein, CNDG is first prepared and tested as an additive for preparing a new engine coolant. The prepared two-dimensional CNDG feature high electrical conductivity, excellent thermal conductivity and heat transfer properties, permitting the CNDG-based water-ethylene glycol coolants to exhibit excellent Mouromtseff number, electrical properties and heat transfer performance for all temperatures and weight concentrations in a car radiator. Further, steady-state forced convective heat transfer experiments have been performed to evaluate the cooling capabilities of the prepared coolants.

Thermal performance of dimpled/protruded circular and annular microchannel tube heat sink

Flow characteristics and heat transfer performance in the circular and annular microchannels with dimples/protrusions are studied numerically. The effects of Reynolds number (Re), dimple/protrusion size, combinations of dimples or protrusions, and arrangement pattern are examined. The results show that in circular microchannels, dimple cases show the poor heat transfer capacity and the staggered dimple case provides no significant difference. The aligned protrusion cases provide excellent heat transfer performance, which is further improved with the increase in protrusion size or Re, and the staggered protrusion case shows the worse heat transfer performance. In annular microchannels, the outer protrusion shows the better heat transfer effect superior to the inner protrusion. In protrusion microchannel, it will be helpful in avoiding the excessive additional flow resistance when dimples are fixed on the opposite wall. The arrangement has no significant effect on the heat transfer and friction characteristic, and the flow separation on the dimple (protrusion) side is suppressed when the protrusion (dimple) is fixed on the opposite wall. From the view point of energy saving, in circular microchannels, the protrusion cases provide the high effectiveness, and the staggered case possesses the lower effectiveness especially in the protrusion case.

An Evaluation of the Application of Nanofluids in Intercooled Cycle Marine Gas Turbine Intercooler

Coolant is one of the important factors affecting the overall performance of the intercooler for the intercooled (IC) cycle marine gas turbine. Conventional coolants, such as water and ethylene glycol, have lower thermal conductivity which can hinder the development of highly effective compact intercooler. Nanofluids that consist of nanoparticles and base fluids have superior properties like extensively higher thermal conductivity and heat transfer performance compared to those of base fluids. This paper focuses on the application of two different water-based nanofluids containing aluminum oxide (Al2O3) and copper (Cu) nanoparticles in IC cycle marine gas turbine intercooler. The effectiveness-number of transfer unit method is used to evaluate the flow and heat transfer performance of intercooler, and the thermophysical properties of nanofluids are obtained from literature. Then, the effects of some important parameters, such as nanoparticle volume concentration, coolant Reynolds number, coolant inlet temperature, and gas side operating parameters on the flow and heat transfer performance of intercooler, are discussed in detail. The results demonstrate that nanofluids have excellent heat transfer performance and need lower pumping power in comparison with base fluids under different gas turbine operating conditions. Under the same heat transfer, Cu–water nanofluids can reduce more pumping power than Al2O3–water nanofluids. It is also concluded that the overall performance of intercooler can be enhanced when increasing the nanoparticle volume concentration and coolant Reynolds number and decreasing the coolant inlet temperature.