Increased Heat Transfer Area Research Articles

Numerical investigation of the energy performance of a guideless irregular heat and mass exchanger with corrugated heat transfer surface for dew point cooling

The paper presents an investigation into the energy performance of a novel irregular heat and mass exchanger for dew point cooling which, compared to the existing flat-plate heat exchangers, removed the use of the channel supporting guides and implemented the corrugated heat transfer surface, thus expecting to achieve the reduced air flow resistance, increased heat transfer area, and improved energy efficiency (i.e. Coefficient of Performance (COP)) of the air cooling process. CFD simulation was carried out to determine the flow resistance (K) factors of various elements within the dry and wet channels of the exchanger, while the ‘finite-element’ based ‘Newton-iteration’ numerical simulation was undertaken to investigate its cooling capacity, cooling effectiveness and COP at various geometrical and operational conditions. Compared to the existing flat-plate heat and mass exchangers with the same geometrical dimensions and operational conditions, the new irregular exchanger could achieve 32.9%–37% higher cooling capacity, dew-point and wet-bulb effectiveness, 29.7%–33.3% higher COP, and 55.8%–56.2% lower pressure drop. While undertaking dew point air cooling, the irregular heat and mass exchanger had the optimum air velocity of 1 m/s within the flow channels and working-to-intake air ratio of 0.3, which allowed the highest cooling capacity and COP to be achieved. In terms of the exchanger dimensions, the optimum height of the channel was 5 mm while its length was in the range 1–2 m. Overall, the proposed irregular heat and mass exchanger could lead to significant enhanced energy performance compared to the existing flat-plate dew point cooling heat exchanger of the same geometrical dimensions. To achieve the same amount cooling output, the irregular heat and mass exchanger had the reduced size and cost against the flat-plate ones.

Influence of surface roughness and porosity of inclusion in water droplet on heat transfer enhancement

Using high-speed camera, the experiments were performed to research evaporation of 10 μl water droplets containing 2 mm solid inclusions in the shape of cube, when heated (up to 850 K) in combustion products of technical ethanol. Adding solid inclusions in water droplets allowed considerably decreasing (by 70%) their evaporation times. Also, the artificial irregularities (roughness and porosity) at the surfaces of solid inclusions were manufactured to increase heat transfer area. Such approach enabled to decrease evaporation times of heterogeneous liquid droplets in high-temperature gases by 40% (when comparing inclusions with artificial irregularities and smooth surface).

Thermal performance evaluation of vertical U-loop ground heat exchanger using in-situ thermal response test

An in-situ thermal response test is applied to the thermal performance evaluation of the vertical closed U-loop ground heat exchangers (GHX). A line source method is used to derive the thermal conductivity and borehole thermal resistance from data obtained by the thermal response test. Tests are made for four types of GHXs with different borehole configurations. Results are presented in terms of configuration and operating parameters of GHXs to investigate their effects on the thermal performances.Results show that while thermal conductivity moderately increases with the increase of heat transfer area either by the use of larger diameter of tube or insertion of more tubes, the borehole thermal resistance and the temperature entering the ground source heat pump (GSHP) are considerably reduced. Of the borehole thermal resistance components evaluated, the grout thermal resistance is the most governing one in the borehole heat transfer (more than 65% of the total borehole resistance), while convective thermal resistance in the tube is almost negligible (less than 2.5%).

Influence of chimney width on the natural convection cooling of a vertical finned plate

We investigated natural convection heat transfer for a finned plate inside a chimney for application in reactor cavity cooling systems (RCCSs). To achieve a large Rayleigh number, the mass transfer rates were measured rather than the heat transfer rates, exploiting the analogy between heat and mass transfer systems. Experiments were carried out with systematically varied fin heights, fin spacings, chimney heights, and chimney widths, and for large Rayleigh numbers (of up to 2.91×107). The experimental results were consistent with the existing correlations, which were based on numerical models. Larger heat transfer rates were observed for larger fin heights and smaller fin spacings due to the increased heat transfer area. As the distance between walls and the fin tip increased, the heat transfer rate decreased, until the chimney effect was no longer observed, where the plume circulated within the duct. An empirical correlation was derived using the test results. The study concludes that the optimizations of chimney parameters as well as fin parameters are required for RCCS applications.

Flow boiling of refrigerant in horizontal metal-foam filled tubes: Part 2 – A flow-pattern based prediction method for heat transfer

For enriching the heat transfer coefficient database of refrigerants flow boiling in metal-foam filled tubes, the heat transfer characteristics of two-phase flow in horizontal metal-foam filled tubes were investigated for flows in small diameter tubes and at higher mass fluxes. The results show that the heat transfer enhancement due to metal foam at lower mass fluxes is about 50% greater than that at higher mass fluxes. A flow pattern based prediction method was developed for a much wider range of working conditions than previous ones. The influence of the metal foam on the flow pattern and heat transfer area increase is both considered in the new prediction method. The heat transfer coefficients predicted by the correlation agree well with experimental data from the present and previous studies.

A Review on Working Pair Used in Adsorption Cooling System

Adsorption cooling system find its application in refrigeration, air conditioning, chiller, Ice making, etc. It uses thermal energy as driving force. Adsorption systems are environmental friendly (zero global warming potential and ozone depleting potential) and also eliminates use of compressor and minimize vibration problem. So it can be used as substitute for conventional vapor compression refrigeration system or vapor absorption system. The adsorption generally classified in two types as physical adsorption (due to weak van der waal forces) and chemical adsorption (chemical reaction between adsorbent and adsorbate form new molecules). The working pair of adsorber and adsorbate play vital role in the performance of adsorption system. Activated carbon, zeolite, silica gel are commonly used adsorber and water, ammonia, methanol and ethanol can be used as adsorbate. The poor heat and mass transfer performance of adsorption is major challenge for researchers. The heat transfer performance of adsorption system can be increased by increasing heat transfer area of adsorber bed i.e., design of new adsorber bed, while mass transfer performance is improved by use of new adsorbent with higher sorption rate. Composite adsorber solve the problem of heat and mass transfer performance of chemical adsorbents and adsorption quantity of physical adsorbents by combination of chemical and physical adsorbent but it can add some limitation with it. In this paper, various adsorption pair, their selection, design of adsorber bed, methods to improve thermal performance of adsorber bed is reviewed with their properties, advantages and limitations.

English

Solar drying is one of the promising methods of reducing post-harvest losses in rural areas. Studies have shown that, heat transfer mechanisms in a solar collector influences the performance of solar dryers. This study aims at improving heat transfer in flat plate solar collectors by designing different air flow patterns inside the collector. Three flat plate solar collectors were constructed by using Pterocarpus timber (Mninga) and tested for their effect in heat transfer at various flow patterns. Three different flow patterns namely: single duct front pass, double duct parallel flow and double duct counter flow were designed and tested. Experimental results show that collector efficiency of single duct front pass, double duct front pass and double duct counter flow were 30.6, 36.1 and 38.2% respectively. It was found that, double duct flow gives improved performance compared to single duct flow due to the increased heat transfer area. In additional, double duct counter flow showed superior performance compared to double duct parallel flow due to extended heat transfer area and the advantage of air preheating at the inlet which reduces heat losses through glazing. Through this study, it was concluded that, solar collector designs with double duct counter flow can improve collector performance for up to 8.3% compared to single duct front pass.   Key words: Single duct front pass, double duct parallel flow, double duct counter flow, energy, solar intensity

ADSORPTION CHARACTERISTICS AND HEAT OF ADSORPTION MEASUREMENTS OF R-134a ON GRANULAR ACTIVATED CARBON

The objective of this study is to investigate the adsorption characteristics of granular activated carbon GAC/R-134a pair in the temperature range of 20–60°C and pressure up to 10 bars. The Dubinin–Astakhov adsorption equilibrium model is fit to experimental data with acceptable error limit. The pressure–temperature–concentration (P–T–X) diagram of the pair is also presented. The isosteric heat of adsorption of R-134a on AC has been calculated using the Clausius–Clapeyron equation as a function of adsorption capacity. The maximum adsorption capacity was found to be 1.92 kgR134a/kgcarbon at 20°C after 1200 s. The experimental results also shows that the increase of heat transfer area improves the adsorption capacity per kg of adsorber, which lead to the design of a finned tubes heat exchanger adsorption unit.

다양한 배플 인자에 따른 셀-튜브 열교환기의 열전달 및 압력강하 특성에 관한 수치해석

Abstract: In numbers of kinds of heat exchanger, the shell-tube heat exchanger is the most commonly used type of heat exchanger in the industry field. In order to improve the thermal performance of the heat ex-changer, this study was analyzed heat transfer characteristics according to arrangement of baffle and direction of baffle and bump phase of baffle about shell-tube heat exchanger using appropriate SST (Shear Stress Transport) turbulence model for flow separation and boundary layer analysis. As the boundary condition for CFD (Computational Fluid Dynamics) analysis, the inlet temperature of shell side was constantly 344 K and the variation of the water flow rate was 6, 12, 18 and 24 l/min. As the result of analysis, zigzag baffle ar-rangement enhances heat transfer rate and pressure drop. Furthermore, in the direction of the baffle, heat transfer rate is more improved with vertical type and angle 45° type than existing type, and pressure drop was little difference. Also, the bump shape of baffle surface contributes to heat transfer rate and pressure drop improvement due to the increased heat transfer area. Through analysis results, we knew that the increase of the heat transfer was influenced by flow separation, fluid residual time, contact area with the tube, flow rate, swirl and so on.

Possibilities of intensifying heat transfer through finned surfaces in heat exchangers for high temperature applications

High temperature heat transfer application actually represents the case of a heat exchanger operated within a process with high temperature. In every industrial domain, a different value of temperature may be considered “high”. We are active in the field of chemical, petrochemical, waste-to-energy, power and process energy recovery heat transfer applications. Here, tube-fin exchangers are successfully used for gas or liquid and/or aggressive fluids with temperatures up to 350 and/or 400 °C. They are also frequently used in combustion systems with air preheating applications. Tubular heat exchangers, especially those with U-tubes, helical and straight tubes are most frequently used for high-temperature applications with temperatures above 650 °C.Extended surfaces are used as an intensification approach to decrease the area requirements on flue gas side. Selection of an extended surface depends on the type of fuel being burned. Generally speaking, enhanced surfaces are used for gaseous media with low heat transfer coefficient. Fins substantially enhance the heat transfer area and consequently heat duty of the equipment.This paper describes this “passive” technique to enhancement of heat transfer in more detail and presents novel types of longitudinally finned tubes intensifying heat transfer by increasing heat transfer area and heat transfer coefficient. This means that the fins not only increase heat transfer area but also make the fluid flowing around them change flow direction, i.e., they increase turbulence. This consequently increases film heat transfer coefficient on fin side.

Experimental study on the spiral and oval spiral EGR cooler efficiencies in a diesel engine

The cooled exhaust gas recirculation (EGR) system is one of the most effective techniques currently available for reducing nitrogen oxide emissions in diesel engine. Because the combustion characteristics in diesel engine involves high temperature and load, the amount of particulate matter (PM) emission tends to increase, thereby the PM fouling in EGR cooler degrades the heat transfer performance considerably, which in turn has a significant influence on the design of the EGR cooler. In the present study, engine dynamometer tests are performed to investigate the influences of PM fouling on the heat exchange characteristics of spiral and oval-spiral type EGR coolers equipped with a diesel engine. The evaluation test results show that the oval-spiral type EGR cooler has higher efficiency by approximate 10 % than the spiral type EGR cooler because of the increase of heat transfer area and the increased removal of PM from the deposit layer due to fluid shear force.

An experimental study on the pressure drop and heat transfer through straight and curved small diameter tubes

A tube type heat exchanger is often the only solution when minimum pressure loss is a requirement. In addition, small diameter tubes are preferable because of an increased heat transfer area within an acceptable pressure loss limit. The present work reports on both an analytic model and experimental results with regards to the pressure drop and heat transfer characteristics of compact straight, C-curved, and U-curved tubes. The inner diameter of the tube (D) for our selected heat exchanger type was 1.26 mm with a thickness of 0.12 mm and a total length of 150.8 D. For the experiment, pressurized nitrogen gas bottles were used rather than an air compressor system in order to simplify the test facility. Hence the pressure conditions were easily set at 10, 30, and 50 bar corresponding to a range of Reynolds numbers from 10000 to 50000. To elevate the air temperature outside the tube (from 100°C to 400°C), an electric furnace was installed around the “test tube”. An analytic model to determine the pressure loss through curved tubes-referred to as the modified friction factor- is proposed. Good agreement was found between the modified friction factor and existing correlations, thus confirming the suitability of this model for determining pressure losses for different shape of tubes. The average measured Nusselt numbers were within 10- 15% of the Dittus-Boelter and Gnielinski correlations.

Heat dissipation structure research for rectangle LiFePO4 power battery

Under hard acceleration or on a hill climb of (hybrid) electronic vehicles, the battery temperature would increase rapidly. High temperature decreases the battery cycle life, increases the thermal runaway, and even causes a battery to explode, that making the management of battery temperature an important consideration in the safety using of electronic vehicles. A study of increasing heat transfer area from the beginning design phase has been conducted to determine and enhance the heat dissipation on the battery surface. Both experiment and simulation methods were used to analyze the cooling performance under identical battery capacities and heights. Optimal external dimensions and cell sizes with the consideration of better battery workability was obtained from the analysis. The heat transfer coefficients were investigated in order to regulate the battery temperature under safety operating range. It was found that the temperature of the experiment battery would be controlled under safety critical when the cell was designed for 180 mm × 30 mm × 185 mm sizes and the surface heat transfer coefficient was 20 W m−2 K−1 at least.

Flow and Heat Transfer Characteristics in Latticework Cooling Channels With Dimple Vortex Generators

A comparative experimental and numerical study has been conducted on the flow and heat transfer characteristics in a latticework cooling channel with U-shaped subchannels combined with dimple vortex generators over the Reynolds number range of 7700–36,985. The average Nusselt number and friction factor of the latticework channel have been obtained. The comparisons between the experimental and numerical data have shown that the numerical computation model can reasonably well predict the heat transfer and pressure loss in the latticework cooling channels. Additional numerical computations were further performed to investigate the effects of subchannel configurations on the flow and heat transfer in the latticework channel, and three different subchannel configurations were studied, which are the dimpled U subchannel, U subchannel, and rectangular subchannel. The experimental data of the heat transfer and pressure loss of the latticework channel with dimpled U subchannels have also been compared with those of the ribbed channels and pin fin channel from the literature. The present study indicated that the superior heat transfer enhancement capability of the latticework cooling is mainly due to the remarkably increased heat transfer area, turning effects producing strong vortical flow in the subchannels, and the interactions between the flow in the crossing subchannels, as well as the interactions between the flow and the crossing ribs on the opposite side.

Numerical study of effect of operating and design parameters for design of steam reforming reactor

A numerical study on the design of a steam reforming reactor consisting of several reforming tubes and one burner is conducted with respect to various operating and design parameters such as GHSV (gas hourly space velocity), input heat capacity, catalyst layer length, and number of tubes. The calculation of the reforming reaction rate is coupled with a three-dimensional heat and mass transfer calculation. It is shown that a large temperature gradient exists in the reforming reactor, resulting in significant variation of the gas temperature and reaction rate along the reforming tube. The reduction of the catalyst layer length induces a decrease in H2 (hydrogen) concentration as well as pressure loss. An increased number of tubes leads to better system efficiency owing to the enhanced heat transfer to the reforming tube. Consequently, to improve the system efficiency and reduce the pressure loss, an increase in heat transfer area and decrease in catalyst layer length should be essential design considerations.

HEAT TRANSFER ENHANCEMENT OF SMALL SCALE HEAT SINKS USING VIBRATING PIN FIN

Heat sinks are widely adopted in electronics coolin g together with different technologies to enhance t he cooling process. For the small electronics applicat ion, the small scale pin fins heat sinks are extens ively used to dissipate heat in electronics devices. Due to the limit of space in the small devices, it is i mpossible to increase heat transfer area. In order to improve the heat transfer performance, the applying the fo rced vibration is one of challenging method. This study applies the vibration frequency between 50 to 1,000 Hz to pin fins heat sinks. The results of numerical si mulation clearly show satisfied heat transfer augme ntation. However, the Pressure drop significantly increases with frequency. This phenomenon affects the heat tr ansfer enhancement performance that it increases with freq uency until certain value then it drops rapidly. Th e results of this study can help designing heat sinks for ele ctronics cooling by employing the concept of vibrat ion.

THE STUDY OF EVACUATED SOLAR COLLECTOR USING PULSATING HEAT PIPE

Solar heat energy, one of the typical renewable energy absorbs radiation energy from the sun and converts it into heat energy. Since it is sustainable and eco-friendly, it can be a good alternative to solve the problem of the depletion of fossil fuels. But it has low density. So, solar collector is very important to use more efficiently. From the viewpoint of low cost as well as short construction schedule for the manufacturing of heat exchanger, flexibility, and high performance, Pulsating Heat Pipe (PHP) draws special attention. Therefore, we use PHP for increasing efficiency of solar collector. In this study, we applied two types PHP to solar collector. One is U type, and other is Double U type. We made evaporating section of PHP by using a ‘U’ shape to prevent flooding which is the problem of conventional ‘I’ type heat pipe of solar collector. Condensing section of PHP was double tube which has the circular finned water tube inside for increasing heat transfer area. PHP was charged R-141b about 100% of evaporating section volume. And then, we compare the solar collector efficiency according to the evaporating section shape of the PHP on standard water flow rate. Consequently, heat collector efficiency of the U type solar collector is about 15% higher than Double U type solar collector. And both solar collector works reliably on the standard water flow rate.

Parametric study on the dynamic behavior of natural convection from horizontal rectangular fin arrays

A parametric study on the dynamic natural convection from long horizontal fin arrays (L=128, 254 and 380mm) is made using a 3-D unsteady numerical analysis. The investigated range of height H is 6.4–38mm and that of spacing S is 6.4–20mm. Increasing L tends to enhance the oscillating sliding-chimney flow, whereas increasing S weakens it. The time-averaged overall convection heat transfer coefficients h¯ are larger for high and short fin arrays, due to the stronger buoyancy and thinner boundary layers. The h¯–S relation exhibits a steep drop when S is narrowed below a threshold, which is larger for lower and longer fins. The optimum fin spacing Sopt occurs near the threshold S, below which the benefit of increasing heat transfer area surrenders to the decrease of h¯ caused by excessive viscous drag. The heat transfer rate per unit base area Q/Ab decreases with increasing L and decreasing H. The predicted dependence of Sopt on H and L agrees well with experimental results and is explained based on numerical results of flow and heat transfer characteristics. The present predictions of Nu agree well with the correlations in the literature which use S as the characteristic length.

Flow and heat transfer characteristics of single jet impinging on protrusioned surface

Combined PIV and numerical simulation, the flow characteristics of a single jet impinging on a protrusioned surface were investigated. And the heat transfer characteristics were explored on the basis of the numerical results. The flow structures of the single jet impinging on protrusioned surface in different cases are similar. The jet flow impinges perpendicularly onto the protrusion and then turns its flow direction. Mini flow separation occurs when the flow passes the edge of the protrusion and secondary impinging jet forms at the same time. The wall jet continues to develop along the radial direction after apart from the edge of the protrusion. Comparison with the case of smooth target surface shows that local Nusselt number increase occurs in the presence of protrusion. The local Nusselt number increases as protrusion relative depth increases. The average Nusselt number increases with protrusion relative depth and jet Reynolds number. For most of the cases, both heat transfer enhancement within the stagnation region and increase of heat transfer area result in overall heat transfer improvement of the jet impingement with protrusioned target surface.

Maximisation of heat transfer in a coil in tank PCM cold storage system

Thermal energy storage systems for both heat and cold are necessary for many industrial processes. High energy density and high power capacity are desirable properties of the storage. The use of latent heat increases the energy density of the storage tank with high temperature control close to the melting point. Tube in PCM tank is a very promising system that provides high packing factor. This work presents an experimental study of a PCM tank for cold storage applications. Two different configurations and different flow rates of the heat transfer fluid were studied. The effectiveness of the PCM storage system was defined as that of a heat exchanger. The results showed that the heat exchange effectiveness of the system did not vary with time, decreased with increasing flow rate and increased with increasing heat transfer area. The effectiveness was experimentally determined to only be a function of the ratio m ˙ / A. This equation was found to be adequately be used to design a PCM storage system, and a case study is presented. It was shown that the tube in tank design together with a low temperature PCM is suitable as a thermal storage facility for cold storage.