High-performance Heat Exchangers Research Articles

Grafted Nanofilms Promote Dropwise Condensation of Low-Surface-Tension Fluids for High-Performance Heat Exchangers

Summary We have designed grafted polymer coatings using initiated chemical vapor deposition (iCVD), a solvent-free, substrate-independent, vapor-phase deposition process, that are capable of stable dropwise condensation of low-surface-tension fluids. Our iCVD coatings effectively increase the vapor-side heat transfer coefficient of the condenser, are stable under industrial environments, and can be applied in a scalable fashion. We explore how tuning the grafting of these iCVD films can minimize the contact angle hysteresis and impact droplet shedding and ultimately heat transfer performance. We measure a 4- to 8-fold enhancement in vapor-side heat transfer coefficients on a variety of substrates including industrially relevant heat exchanger metals such as steel and titanium. We demonstrate that iCVD can be applied to tubular geometries, which allows for direct translation to shell-and-tube heat exchangers, used across industries. Finally, we estimate the overall cycle efficiency improvement our coatings can provide to such processes using an example organic Rankine cycle.

Heat transfer in heat exchangers from a ribbed surface under conditions of asymmetric temperature of the bases of collective finning

The solution of the problem of heat flow in heat exchangers from the surface of collective finning at different temperatures of its ends is given. The obtained analytical formulas allow us to quantify the influence of the asymmetry of the boundary conditions on the heat transfer through the finned heat exchange surface. Calculated dependencies can be used in the design of high-performance heat exchangers for power engineering, agricultural process equipment, air conditioning systems, ventilation and heat supply.

Oriented magnetic field effect on mixed convective flow of nanofluid in a grooved channel with internal rotating cylindrical heat source

In this paper, the flow and heat transfer characteristics for the influence of oriented magnetic field on mixed convection flow of water based nanofluid inside a grooved channel with a rotating heat source are numerically investigated. The channel is cooled from the grooved as well as vertical walls and heated from the bottom walls as well as rotating heat source while the remaining walls are thermally insulted. The channel is permeated by an inclined magnetic field of uniform strength, and a modified model of effective thermal conductivity is used to improve the overall thermal conductivity of nanofluids. The governing partial differential equations representing the flow model are solved with Galerkin weighted residual finite element method. A complete parametric study is carried out based on numerical results to show the variations of flow and temperature fields in terms of streamlines, isotherms, velocity and temperature profiles, average Nusselt number and average temperature for the effects of pertinent parameters including Reynolds number, Hartmann number, volume fraction of nanoparticles and inclination angle of the magnetic field. It is observed that average heat transfer rate enhances noticeably with the increase in Reynolds number and volume fraction and reduces for increasing Hartmann number. It is also found that the augmentation of heat transfer due to higher inclination angle of magnetic field becomes significant when the value of Hartmann number is sufficiently large. Moreover, the effects of governing parameters on the fluid flow and heat transfer behaviors are affected remarkably with the presence of rotating heat source and the direction of rotation as well. Comparisons of the present results with the previous published results are performed and excellent agreement is found. The outcome of this study can be applied to design engineering equipments such as high performance heat exchangers, cooling of electronic devices and circuit boards, cooling of nuclear reactors and biomedical equipments, etc.

Large eddy simulation analysis of the heat transfer enhancement using self-oscillating fluidic oscillators

Using an unsteady jet created by the fluidic oscillator represents many opportunities to enhance the heat removal performance of impinging jet. Numerical simulations were conducted to compare the heat transfer performance of two fluid oscillators and a direct jet with unsteady Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) turbulence models implemented. Time-resolved and time-averaged flow fields and heat transfer results were presented. Simulation results showed that the fluid dynamic and heat transfer performance predicted by the RANS and LES models matched well for different Reynolds numbers.The self-oscillating impinging jets were beneficial in enhancing the heat removal performance by increasing the average Nusselt numbers and covering a larger impinging cooling surface. The wide impinging zone and significant heat transfer enhancement suggested it was promising to implement self-oscillating impinging jet concept into existing cooling technologies, such as multi-phase cooling and thin film cooling, for the next-generation high-performance heat exchangers.

Bio-inspired self-agitator for convective heat transfer enhancement

Convective heat transfer plays an important role in both the fundamental research and the development of high-performance heat exchangers. Inspired by blades of grass vibrating in the wind, we developed a self-agitator for convective heat transfer enhancement. Because of fluid-structure interactions, the agitator, with self-sustained vibration, can generate strong vortices to significantly break the thermal boundary layer and improve fluid mixing for enhanced convective heat transfer. In particular, we establish a methodology to link the vorticity field at a preferred frequency to the optimal improvement in the convective heat transfer. To identify the self-agitator preferred frequency, mode analysis is performed with simulation results using dynamic mode decomposition. Experimental results are also obtained to further validate the proposed approach. These results show that the proposed self-agitator design can improve the convective heat transfer by 120% in a conventional heat exchanger without additional pumping power requirements and can achieve a Nusselt number of up to 30 within the laminar flow region, which is improved by 200% with the same Reynolds number compared to the clean channel.

Additively manufactured high-performance counterflow heat exchanger

The purpose of this article is to demonstrate that additive manufacturing is a viable method for producing counterflow heat exchangers that have a very high power to volume ratio. For this study, a heat exchanger with 144 flow channels in a checkerboard pattern was designed and additively manufactured from AlSi10Mg. The heat exchanger was tested by measuring the heat transfer between two liquids in a counterflow set-up, where it reached exceptionally high performance when considering its volume and weight. The heat transfer properties of the heat exchanger were verified analytically through calculations, which identified that the high surface roughness of the channels provides a significant improvement in heat transfer properties. The heat transfer capabilities were measured on two separate occasions to investigate the possible change of properties of additively manufactured heat exchangers over time when used with tap water. A moderate decrease in heat flow and increase in pressure drop were noted between the measurements. The deterioration of heat transfer capabilities could present a significant challenge for additively manufactured heat transfer applications and will be closely examined in future research.

Thermal and hydraulic numerical study for a novel multi tubes in tube helically coiled heat exchangers: Effects of operating/geometric parameters

Compactness with high performance heat exchangers are main challenges in a lot of engineering applications. Thus, this research reports CFD simulation of a novel MTTHC (multi tubes in tube helically coiled) heat exchanger using ANSYS-FLUENT 14.5. The aim of the work is to investigate the thermal and hydraulic performance of the MTTHC for turbulent flow. The effects of the operating and geometrical parameters of the coil on the cold/hot water Nusselt numbers, heat transfer coefficients, pumping power, effectiveness, and thermal-hydraulic index are studied and presented. The results show that, the largest heat transfer coefficient is found at N = 3 and β = 0° & 90°, and the pumping power (P) rises with ≅20 times if N changed from 1 to 5 at any β. Moreover, the effectiveness of the coil (ε) has the largest values at β = 0° & 90° and N=3, and it enhances with 8.5%, 9% and 7% if N increased from 1 to 3 at β = 0°, 45° and 90°, respectively. In addition, thermal-hydraulic index (ξ) improves with 5%, 8% and 6% if N increased from 1, to 3 at β = 0°, 45° and 90°, respectively. Finally, Numerical correlations for P, ε and ξ are correlated and presented within reasonable errors.

Perforated plate convective heat transfer analysis

The need for high performance heat exchangers has led to the development of many types of surfaces that enhance the rate of heat transfer. One of the heat exchanger types are the matrix heat exchangers. Matrix heat exchangers consist of a series of perforated plates, separated by a series of spacers. The present study experimentally investigates the overall heat transfer characteristics of flow through a perforated plate with 2 mm in diameter, the hole length to diameter ratio of 1 and 25.6% porosity. Numerical simulations were performed to determine the overall heat transfer in the function of geometric parameters. Reynolds numbers based on the perforated plate pitch were set in the range from 50 to 500. The results of the average Nusselt number prediction were compared with the related experimental correlations. The experimental data agreed on qualitatively with the results obtained using a CFD. Using these data, a Nusselt criterial equation was obtained.

Thermal performance of brazed metalfoam-plate heat exchanger as an evaporator for organic Rankine cycle

Compact-sized organic Rankine cycle (ORC) power generators call for higher performance and down-sized heat exchangers. Heat exchangers in ORC systems, especially evaporators contribute to a big portion of the system size as well as the capital cost, and their price is also directly related to their size. Since the heat transfer area plays a direct role in the performance of heat exchangers, high-porosity metal foams are proposed for insertion into heat exchanger channels to enhance the heat transfer mechanism in evaporators. Their high surface area to volume ratio makes them a good candidate for manufacturing high-performance heat exchangers. The metalfoams are being considered to improve performance while keeping the size of heat exchangers small. In this experimental study, the performance of a 100 kW class heat exchanger with the channels brazed with nickel based metal foam and stainless steel sheets was investigated. A hot water loop was designed for heat input. The cold side of the heat exchanger works with R245fa as the working fluid. The phase-change heat transfer experiments were performed with different mass flow rates ranging from 0.32 to 0.51 kg/s while the operating pressure was at 10 to 13 bar with hot water inlet temperature was 133℃. Furthermore, the heat transfer performance is compared with a commercial plate heat exchanger (produced by Alfalaval, AC120EQ) which was custom-made as a 100kW class evaporator. In the comparison test, refrigerant side has 0.5kg/s of mass flow rate at 44℃ and heat source side has 1.23 kg/s mass flow rate at 132℃. Although the pressure drop in the metal foam plate heat exchanger is increased compared to that of the conventional plate heat exchanger, increase of the recovered waste heat from the heat source is much higher due to higher overall heat transfer coefficient. As a result, the energy density of the new heat exchanger is about 2.5 times higher than that of the conventional plate heat exchanger.

Design optimization and validation of high-performance heat exchangers using approximation assisted optimization and additive manufacturing

The airside thermal resistance of air-to-fluid heat exchangers dominates the overall thermal resistance. On conventional heat exchanger's, fins are required to address such challenges; but their benefits are not limitless and are bound mainly by the tube size and shape. The reduction of the tube characteristic length has favorable impact on compactness and heat transfer. Conventional tubes are typically limited to round, elliptical or flat shapes which result in particular thermal-hydraulic characteristics. The current article has three main objectives. First, discuss the importance of fins on typical air-to-fluid heat exchanger's and how they become unattractive at smaller characteristic lengths with numerical analyses to support this argument from different perspectives. Second, present a proof-of-concept design with small finless tubes and a novel shape that can outperform a microchannel heat exchanger. Third, present a comprehensive analysis with shape optimization leveraging automated computational fluid dynamics simulations and approximation assisted optimization techniques. Optimum designs can achieve more than 50% reduction in size, material, and pressure drop compared to the baseline microchannel heat exchanger. The method is validated with the experimental validation of a metal three-dimensional printed prototype of the NTHX-001. The numerical simulations agreed within less than 5% in capacity, 10% in air heat transfer coefficient, and 15% in air pressure drop.

A 6-focus high-concentration photovoltaic-thermal dish system

Abstract We present the design, optical characterization and full-system modeling of a novel 6-focus, high-concentration photovoltaic-thermal solar polygeneration system, aiming at an energy-efficient and cost-effective utilization of the solar resource. Essential to this system is a compact, modular solar dish concentrator design optimized for mass-production, structural rigidity, and scalability, with a high geometric concentration ratio of 1733× at each of its six receivers. Every receiver comprises 36 triple-junction CPV cells, interconnected in a unique hybrid parallel-serial scheme that mitigates mismatch losses caused by non-uniform irradiance distributions. Cogeneration is enabled by using high-performance microchannel heat exchangers, allowing the extraction of low grade heat for secondary thermal processes. The tested prototype achieves an average solar radiative flux of 1374 suns on each of the receivers. By optimizing several design parameters, the CPV-thermal system can deliver a solar-to-electricity conversion efficiency of 28.5% in PV-only mode and 26.6% in cogeneration mode while extracting heat at 89.8 °C, and a power of 12.1 kW el and 11.3 kW el /21.5 kW th respectively, matching the performance of state of the art CPV commercial systems, while striving towards a reduction of the investment costs.

Experimental stand for investigation of fluid flow in heat exchangers with cross-flow arrangement

The operation analysis of high-performance heat exchanger with tubes elliptical indicated that the heat exchangers can be subject to damage. The reason for this is probably improper distribution of working fluid in tubular space of heat exchanger. Therefore, a part of the tubes may be improperly cooled and subject to compressible stresses. The paper presents an experimental stand allowing to confirm the given assumption. The experimental investigation enables to examine the mass flow rate in heat exchanger tubes. Also, it is possible to assess the impact of the construction of inlet, intermediate and outlet chambers on the flow distribution within the heat exchanger tubes.

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.

An Advanced Energy System Using a Small Fast Reactor as an Energy Source

An advanced energy system has been proposed that involves a supercritical carbon dioxide gas turbine fast reactor (S-CO2 FR) as a dispersed energy source, a new waste-heat recovery system from the FR, and a bioconversion system using the recovered waste heat. The FR with S-CO2 gas turbine achieves higher cycle efficiency than conventional sodium-cooled FRs with steam turbines, eliminating problems of conventional FRs related to safety, plant maintenance, and construction costs. The S-CO2 FR consumes minor actinide elements produced in light water reactors as fuel, thereby reducing long-lived radioactive wastes and environmental loads imparted by long-term geological disposal. The recovered waste heat is used for methane and methanol production through fermentation of human and animal wastes in cities and farms. The methane and methanol are easily transported and can accommodate demand changes; they are useful as fuel of fuel cells, automobiles, and gas turbine power plants. The total energy utilization efficiency in electricity and heat is estimated to be higher than 85%, contributing to saving of energy resources and reduction of greenhouse-gas emissions. Consumption of the waste products of cities and farms for production of methane and methanol fosters a recycling society. Compact and high-performance microchannel heat exchangers are used in the S-CO2 FR, the new waste-heat recovery system and the fermentation system.

Flow boiling visualization and heat transfer in metal-foam-filled mini tubes – Part I: Flow pattern map and experimental data

High-porosity open-cell metal foams are well known to enhance the heat transfer mechanism in rectangular or circular channels. Their high surface area to volume ratio makes them a great candidate for manufacturing high-performance small-scale heat exchangers. This two-part experimental study investigated the two-phase flow boiling inside a circular copper mini tube. In Part I, the flow pattern was visualized by high-speed imaging in glass tubes. Flow pattern maps, the heat transfer coefficient, and pressure drop are presented for mean vapor quality of 0.1–0.7, heat flux of 20–40kW/m2, and mass flux of 400–700kg/m2s. The experiments were also performed without the metal foam in the mini tube for comparison to the original data. In this range of experimental conditions, the metal foam increased the heat transfer coefficient up to 3.2 times. Also, as expected, the metal foams adversely affected the pressure drop inside the tubes.

A new optimization approach for shell and tube heat exchangers by using electromagnetism-like algorithm (EM)

This study proposes a new procedure for optimal design of shell and tube heat exchangers. The electromagnetism-like algorithm is applied to save on heat exchanger capital cost and designing a compact, high performance heat exchanger with effective use of the allowable pressure drop (cost of the pump). An optimization algorithm is then utilized to determine the optimal values of both geometric design parameters and maximum allowable pressure drop by pursuing the minimization of a total cost function. A computer code is developed for the optimal shell and tube heat exchangers. Different test cases are solved to demonstrate the effectiveness and ability of the proposed algorithm. Results are also compared with those obtained by other approaches available in the literature. The comparisons indicate that a proposed design procedure can be successfully applied in the optimal design of shell and tube heat exchangers. In particular, in the examined cases a reduction of total costs up to 30, 29, and 56.15 % compared with the original design and up to 18, 5.5 and 7.4 % compared with other approaches for case study 1, 2 and 3 respectively, are observed. In this work, economic optimization resulting from the proposed design procedure are relevant especially when the size/volume is critical for high performance and compact unit, moderate volume and cost are needed.

Boiling Heat Transfer and Pressure Drop of a Refrigerant R32 Flowing in a Small Horizontal Tube

In this study, experiments were performed to examine characteristics of flow boiling heat transfer and pressure drop of a low global warming potential refrigerant R32 flowing in a horizontal copper circular tube with 1.0 mm inside diameter for the development of a high-performance heat exchanger using small-diameter tubes or minichannels for air conditioning systems. Axially local heat transfer coefficients were measured in the range of mass fluxes from 30 to 400 kg/(m2·s), qualities from 0.05 to 1.0, and heat fluxes from 2 to 24 kW/m2 at the saturation temperature of 10°C. Pressure drops were also measured in the rage of mass fluxes from 30 to 400 kg/(m2·s) and qualities from 0.05 to 0.9 at the saturation temperature of 10°C under adiabatic condition. In addition, two-phase flow patterns were observed through a sight glass fixed at the tube exit with a digital camera. The characteristics of boiling heat transfer and pressure drop were clarified based on the measurements and the comparison with data of R410A obtained previously. Also, measured heat transfer coefficients were compared with two existing correlations.

Push-pull Mechanical Ventilation System with Heat Exchanger Controlled by Intake Air Temperature

The purpose of this study is the development of a push-pull ventilation system, in which the air supply and exhaust amount is individually controlled by indoor and outdoor temperature difference, to maximize the ventilation performance and energy efficiency. By individually controlling the air flow rate of the supply and exhaust depending on the demand, the fan operating energy consumption is significantly reduced. High performance heat exchanger is also adopted. During non-heating period, it is possible to reduce indoor chemical contamination occurring with wall infiltration, by halving the amount of exhaust and pressurizing the room.

Two-Phase Flow and Boiling Heat Transfer in Small-Diameter Tubes

For the development of a high-performance heat exchanger using small channels or minichannels for air-conditioning systems, it is necessary to clarify the characteristics of vapor‐liquid two-phase flow and heat transfer of refrigerants in small-diameter tubes. In this keynote paper, the related research works that have already been performed by the author and coworkers are introduced. Based on the observations and experiments of R410A flowing in small-diameter circular and noncircular tubes with hydraulic diameter of about 1 mm, the characteristics of vapor‐liquid two-phase flow pattern and boiling heat transfer were clarified. In low quality or mass flux and low heat flux condition, in which the flow was mainly slug, the “liquid film conduction evaporation” heat transfer peculiar to small-diameter tubes prevailed and exhibited considerably good heat transfer compared to nucleate boiling and forced convection evaporation heat transfer. The effects of the tube cross-sectional shape and flow direction on the heat transfer primarily appeared in the region of the “liquid film conduction evaporation” heat transfer. A new heat transfer correlation considering all of three contributions has been developed for small circular tubes.

Comparison of metal foam heat exchangers to a finned heat exchanger for low Reynolds number applications

Due to its high porosity and large specific surface area, open-cell metal foam is an attractive material for heat transfer applications. In this article the performance of metal foam heat exchangers is compared to the performance of a bare tube bundle and the performance of an existing conventional louvered fin heat exchanger. A macroscopic model consisting of the Darcy–Forchheimer–Brinkman flow model and the thermal non-equilibrium energy model is used to perform two-dimensional simulations on metal foam heat exchangers. Because thermal design of heat exchangers is always a trade-off between heat transfer and pressure drop, both are considered together when evaluating the heat exchangers’ performance. The foamed heat exchangers show up to 6 times higher heat transfer rate than the bare tube bundle at the same fan power. If the fins are replaced by metal foam while keeping the overall dimensions the same, the finned heat exchanger shows in all cases the best performance. However, a metal foam heat exchanger can outperform the finned heat exchanger if the frontal area is changed. Optimization is required to select the best foam parameters, material and dimensions. This clearly shows the potential of open-cell metal foam for high performance heat exchanger designs.