Coaxial Heat Research Articles

Augmentation of thermal efficiency of the glass evacuated solar tube collector with coaxial heat pipe with different refrigerants and filling ratio

Modified coaxial heat pipes have been designed and manufactured to improve the thermal performance of the glass evacuated solar collectors. Heat pipes were made up of two concentric copper tubes so that the annulus volume space between the concentric tubes was charged with refrigerant. In addition, the air as the working fluid at four different mass flow rates 0.0051, 0.0062, 0.007 and 0.009kg/s flows through the inner tube of the heat pipe to the flow through the annulus between the heat pipe and glass evacuated solar tubes. The effect ofthe tilt angleofthe evacuated tube on thermal performance of the evacuated solar tube collector was examined to obtain the optimum tilt angle during the experiments period. The influence of filling ratio for the two types of refrigerant R22 and R 134a on the thermal efficiency of the coaxial heat pipe solar collector at filing ratio range from 30% to 60% was conducted experimentally. Results show that the maximum increased in the thermal efficiency reached 67% corresponding to without heat pipes at mass flow rate 0.009kg/s. The experiment results showed similarity between the two refrigerants.

Field evaluation of microencapsulated phase change material slurry in ground source heat pump systems

Microencapsulated phase change material (MPCM) slurries were field-tested in ground source heat pump (GSHP) systems in the Southern United States to validate their thermal performance and durability as heat transfer fluids (HTF). MPCM particles consisted of methyl stearate (melting temperature: 39.5 °C) as phase change material (PCM), microencapsulated with polyurea as shell material. Experimental results showed that MPCM slurries transport more thermal energy than water at a constant pumping power due to the higher heat capacity associated with the PCM. Demonstration experiments showed that using MPCM slurries improved the heat load-to-pumping power ratio by up to 34% when using a coaxial heat exchanger. The coefficient of performance of the GSHP system was enhanced by up to 4.9% when using MPCM slurries. It can be concluded that MPCM slurries are viable HTF because of their higher heat capacity. In terms of durability, no significant damage of MPCM was detected under continuous pumping conditions after 123,252 pump-cycles or an estimated life span of 10.5 years. Results also suggest that progressive cavity pumps are more suitable than centrifugal pumps from the durability point of view. Long-term durability studies should be considered for the eventual implementation of MPCM technology in heating and cooling applications.

Caracterización del IHX en un sistema de refrigeración automotriz y el COP con refrigerante R134A

The present work has the objective of characterizing several coaxial heat exchanger geometries in an automotive cooling system and its effect on the performance coefficient, using R-134a as refrigerant by means of experimental tests. In the first instance, the experimental tests were carried out with different geometries of coaxial interchanges involved in the high-pressure line between the condenser and the expansion valve, as well as the low pressure line between the evaporator and the compressor, Between the volume of the exchanger and the coefficient of performance by means of a multifactorial analysis. The development of the research entails complying with parameters for the acquisition of experimental data and the development of an adequate experimental design. Statistical regressions were developed with R2 of 86.86% and a standard deviation of 2.2 for the low-pressure zone, while for the high pressure zone The regression determined a R2 of 95.17% and a standard deviation of 0.57. As for the COP and the T variables is statistically significant with a value of R2 of 99.97%, thus helping to design the heat exchangers to decrease fuel consumption in vehicles.
 Index Terms— : IHX,COP, R-134a, Cooling cycle Automotive, heat exchangers

Effects of pitch and corrugation depth on heat transfer characteristics in six-start spirally corrugated tube

Spirally corrugated tube is one of the most important parts of coaxial heat exchangers. It can greatly improve the heat transfer efficiency of heat exchangers. Here, a novel spirally corrugated tube with six-start is proposed. However, up to now, there is little literature about the heat transfer performance and flow field of this novel six-start spirally corrugated tube. In this paper, the effects of geometric parameters (pitch p, corrugation depth e), Reynolds number Re and fluid properties on the heat transfer performances are investigated based on the validated numerical model. The results show that with the increasing of pitch p, both the heat transfer coefficient h and Nusselt number Nu decrease gradually. Meanwhile, with the increasing of corrugation depth e, both the secondary flow velocity vxy and the vorticity of longitudinal vortex increase gradually. Moreover, under the same working condition, the heat transfer performances of the six-start spirally corrugated tube are affected by both the working medium and Reynolds number. Finally, a criterion correlation for heat transfer calculation in the six-start spirally corrugated tube is proposed and validated to be reliable and suitable. This work can reveal the enhanced heat transfer mechanism of the six-start spirally corrugated tube and benefit the further research on heat transfer characteristics of multi-start spirally corrugated tube or other related devices.

Thermophysical properties and heat transfer performance of carbon nanotubes water-based nanofluids

In this paper, thermal conductivity and rheological properties of CNT water-based nanofluids were experimentally measured, whereas density and heat capacity were evaluated from appropriate theoretical correlations. The influence of nanoparticle content and base fluids on thermophysical properties of nanofluids was presented and discussed. Then, the thermal performance and convective heat transfer of such nanofluids were investigated in a coaxial heat exchanger working in co-current flow. Fixed wall temperature boundary condition and laminar regime were also considered during the experiments. The results were presented discussing the effect of the entrance region, Reynolds number and nanofluids composition.

Field synergy analysis of six starts spiral corrugated tube under high Reynolds number

Coaxial heat exchanger is widely used in air conditioning, refrigeration etc., due to its highly efficient heat transfer performance. Spiral corrugated tube plays an important role in coaxial heat exchanger. In this paper, the numerical model of a six starts spiral corrugated tube and a smooth tube with the same size are developed. The temperature field and the velocity field of their streamline and longitudinal vortex are investigated respectively. Then, their heat transfer and pressure drop performance inside the spiral corrugated tube under different high Reynolds number is investigated by compared their Nusselt number and friction coefficient. Meanwhile, their field synergy performances with their field synergy angles are presented. The result shows that the Nusselt number and friction coefficient of spiral corrugated tube are always larger than the smooth tube, and with the increasing of Reynolds number, the heat transfer performance of SCT becomes better than smooth tube, however, the friction coefficient ratio also increases synchronously. And in spiral corrugated tube, the field synergy angel is smaller than in the smooth tube. This work can be referred by some who are also dealing with spiral corrugated tube and its heat performance research.

2000m 단일 시추공에서 밀폐 동축 방식 지중 열교환기의 취득온도 성능평가

2000m 단일 시추공에서 밀폐 동축 방식 지중 열교환기의 취득온도 성능평가

Determination of optimum geothermal Rankine cycle parameters utilizing coaxial heat exchanger

The main objective of this study is determining the optimal parameters of geothermal Rankine cycle using coaxial heat exchanger from 4 viewpoints: (1) First law of thermodynamics (2) Working fluid (3) Second law of thermodynamics and (4) Costs. Optimization of diameter ratio was accomplished with respect to pressure drop decrement inside the heat exchanger and cycle thermal efficiency increment. Optimal values for diameter ratio were determined to be 0.675 and 0.353 for pressure drop minimization and thermal efficiency maximization, respectively. The Organic Rankine Cycle which uses water as intermediate fluid was assessed regarding to the optimum values of diameter ratio from energy, exergy and exergo-economic standpoints. The analysis led to exergy destruction cost estimation. The amount of exergy destruction cost was added to the cycle cost function which includes the cost of purchasing, maintenance and piping. To specify the best working fluid for the power cycle, a comparison was made between four different organic fluids and water based on power generation and costs. Finally, R123 was detected as the most appropriate working fluid. For obtaining an optimum system, two standpoints were still remained which should be considered and they are cost and exergy efficiency. These parameters were optimized while the thermal efficiency constraint had been set to its optimal value (for 0.353 as diameter ratio). As a result, optimum values for cycle parameters were met. These parameters are pressure and temperature of main organic fluid and the condenser pressure. For coaxial heat exchanger, the optimized parameters are input water velocity, output steam pressure and outer diameter.

Automatic devices for electrochemical water treatment with cooling of electrolyte

The most common disinfectants for water treatment are based on chlorine and its compounds. Practically, water treatments with chlorine compounds have no alternative, since they provide, in comparison to other effective processes such as ozonization or ultraviolet irradiation, high residual disinfection capacity. Unfortunately, all of chlorine-based compounds for disinfection tend to degrade during storage, thus reducing the concentration of active chlorine. Apart from degradation, additional problems are transportation, storage and handling of such hazardous compounds. Nowadays, a lot of attention is paid to the development of electrochemical devices for in situ production of chlorine dioxide or sodium hypochlorite as efficient disinfectants for water treatment. The most important part of such a device is the electrochemical reactor. Electrochemical reactor uses external source of direct current in order to produce disinfectants in electrochemical reactions occurring at the electrodes. Construction of an electrochemical device for water treatment is based on evaluation of optimal conditions for electrochemical reactions during continues production of disinfectants. The aim of this study was to develop a low-cost electrochemical device for the production of disinfectant, active chlorine, at the place of its usage, based on newly developed technical solutions and newest commercial components. The projected electrochemical device was constructed and mounted, and its operation was investigated. Investigations involved both functionality of individual components and device in general. The major goal of these investigations was to achieve maximal efficiency in extreme condition of elevated room temperature and humidity with a novel device construction involving coaxial heat exchanger at the solution inlet. Room operation of the proposed device was investigated when relative humidity was set to 90% and the ambient temperature of 38?C. The obtained results in such extreme operation conditions reveled that it was possible to obtain required concentration of 0.85% of active chlorine with maximal temperature of outlet electrolyte of 35.3?C, which is within the limits of optimal temperatures.

Heat transfer properties of aqueous carbon nanotubes nanofluids in coaxial heat exchanger under laminar regime

The thermal performance of water-based multi-wall carbon nanotubes nanofluids are measured in a coaxial heat exchanger under laminar regime within the range of Reynolds numbers 500–2500. The convective heat transfer properties with constant wall temperature are evaluated for four different multi-wall carbon nanotubes based nanofluids at low concentration of 0.05% in weight (0.026% in volume). The measurements of thermal and rheological properties of the nanofluids with operating temperature were investigated experimentally. The effects of the aspect ratio of carbon nanotubes, the type of base fluid and surfactant on viscosity, thermal conductivity and laminar convective heat transfer were studied. Based on the experimental results, we reported the shear-thinning behavior of nanofluids, the nanofluid viscosity being dependant on the base fluid type in the Newtonian region. We also showed that the enhancement of the thermal conductivity and the average convective heat transfer of nanofluids increased with the aspect ratio of nanotubes and decreased when the thermal conductivity of the base fluid increases. This enhancement attains at least 10% in comparison to base fluid even with the low content of nanotubes used.

Коаксиальная тепловая труба для охлаждения отражателя лазера

The paper presents the development and research results for a coaxial heat pipe designed for cooling of a reflector of a solid-state laser. A coaxial cylindrical heat pipe, designed to cool the laser reflector, provides that the temperature of the heat-removing surface does not exceed 120°C at any orientation in the gravitational field, if the heat is removed by forced convection of air with the temperature of 60°C in a pulsed mode of heat flow supply of 300 W. Thermal resistance of the developed heat pipe is 0,03 K/W, the specific thermal resistance — 1,1*10–3 m2*K/W. The developed cooling system based on the evaporation-condensation principle, allows ensuring temperature uniformity of the cooling surface at low thermal resistance.

A finite volume coaxial heat exchanger model with moving boundaries and modifications to correlations for two-phase flow in fluted annuli

Coaxial Heat Exchangers (CHXs) are now used extensively in heat pump (HP) and refrigeration systems. The design of such systems requires estimation of CHX's thermal and hydraulic performance. This paper presents a generalized finite volume CHX model that is capable of simulating single-phase and two-phase flow with a smooth or fluted inner tube. The concepts of segment insertion and subdivision (moving boundary within the segment) are adopted in this model to track the phase change point along the flow channel. This allows for reliable model accuracy even with a lower number of discretized finite volumes, thereby reducing computation time. For the segment insertion and subdivision functions, four different approaches are provided in order to achieve the best accuracy and least computational effort under various flow configurations and fluid conditions. The simulation model can be applied to straight and helical tubes with choices of smooth, grooved and fluted tube surface. Empirical single-phase and two-phase correlations from the literature are adopted in the proposed model for the applicable surfaces. At present, there are no correlations for two-phase flow in fluted tube annuli, in the open literature. This paper proposes modifications to existing two-phase fluted surface heat transfer and pressure drop correlation formulations by applying empirical two-phase flow multipliers onto existing fluted tube single phase correlations. The solving methodology of this model requires the heat exchanger geometry, flow configuration, inlet states, as well as mass flow rates of the fluids to solve for the outlet conditions, heat load, pressure drop and charge of the CHX. Coupled with non-linear equation solver, the model can also provide the required mass flow rate for a specific superheat/subcooling on either side of the CHX. The model and modified correlations are validated against experimental data of brine-to-refrigerant fluted tube condenser and evaporator used in a heat pump application.

Residence time distributions in a modular micro reaction system

Residence time distributions (RTDs) were investigated in a retention reactor (RR) and a coaxial heat exchanger (CHE) of a commercial modular micro reaction system, to improve processing time control and implement this system as a lab scale tool for thermal process development.RTD were investigated by step experiments (30g/L NaCl solution against distilled water at 20°C). Given a measured in- and outlet conductivity distribution where the outlet was produced by a convolution with a transfer function E(t), E(t) parameters (mean residence time θ and variance α2) were recovered using a computational routine.The flow in the RR remained close to a plug flow despite laminar conditions. Molecular diffusion was not sufficient to explain this improvement and further modeling would be required to clarify this behavior. The CHE showed a significant dispersion and θ two to five times longer than estimated, possibly due to flow defects associated to its complex geometry.

Design and optimization of a downhole coaxial heat exchanger for an enhanced geothermal system (EGS)

The present study considers the design, performance analysis and optimization of a downhole coaxial heat exchanger for an enhanced geothermal system (EGS). The optimum mass flow rate of the geothermal fluid for minimum pumping power and maximum extracted heat energy was determined. In addition, the coaxial pipes of the downhole heat exchanger were sized based on the optimum geothermal mass flow rate and steady-state operation. Transient effect or time-dependent cooling of the Earth underground, and the optimum amount and size of perforations at the inner pipe entrance region to regulate the flow of the geothermal fluid were disregarded to simplify the analysis. The paper consists of an analytical and numerical thermodynamic optimization of a downhole coaxial heat exchanger used to extract the maximum possible energy from the Earth's deep underground (2 km and deeper below the surface) for direct usage, and subject to a nearly linear increase in geothermal gradient with depth. The thermodynamic optimization process and entropy generation minimization (EGM) analysis were performed to minimize heat transfer and fluid friction irreversibilities. An optimum diameter ratio of the coaxial pipes for minimum pressure drop in both limits of the fully turbulent and laminar fully-developed flow regime was determined and observed to be nearly the same irrespective of the flow regime. Furthermore, an optimum geothermal mass flow rate and an optimum geometry of the downhole coaxial heat exchanger were determined for maximum net power output. Conducting an energetic and exergetic analysis to evaluate the performance of binary power cycle, higher Earth's temperature gradient and lower geofluid rejection temperatures were observed to yield maximum first- and second-law efficiencies.

1002 住宅用基礎杭を用いた同軸二重管式地中熱交換器での自然対流の影響

1002 住宅用基礎杭を用いた同軸二重管式地中熱交換器での自然対流の影響

Borehole resistance and vertical temperature profiles in coaxial borehole heat exchangers

Ground source heat pump systems are often coupled to the ground by circulating a fluid through vertical Borehole Heat Exchangers (BHEs). The design of a system requires estimates of the ground thermal conductivity and the borehole thermal resistance, which are usually determined by an in situ thermal response test on a completed borehole. The usual test interpretation methods average the inlet and outlet fluid temperatures and use this mean temperature as the average temperature along the borehole length. This assumption is convenient but does not strictly apply. For a coaxial heat exchanger this paper develops an analytical model for the vertical temperature profiles, which can be used instead of the mean temperature approximation to estimate borehole resistance. The model is verified with measured temperatures on a BHE, where an optical technique allows continuous measurements along a coaxial borehole during a distributed thermal response test. A sensitivity study shows that the proposed method corrects errors in the mean temperature approximation, which overestimates the borehole resistance in a coaxial borehole.

303 小型二重管式熱交換器による高い温度変動を伴う非圧縮性流体の温度変動安定化(OS4-1 熱および物質輸送現象の基礎と応用1,OS4 熱および物質輸送現象の基礎と応用)

303 小型二重管式熱交換器による高い温度変動を伴う非圧縮性流体の温度変動安定化(OS4-1 熱および物質輸送現象の基礎と応用1,OS4 熱および物質輸送現象の基礎と応用)

Short time-step performances of coaxial and double U-tube borehole heat exchangers: Modeling and measurements

Several authors have investigated the long time-step performance of borehole heat exchangers. In that time scale, the borehole thermal capacitance is generally neglected, since the time span of interest is on the order of months or years. The borehole thermal capacitance consists both of grouting material and heat carrier fluid, and it mostly affects the short time-step behavior, when hourly or shorter time intervals are considered. Some models are available in the literature for short time-step simulations of borehole heat exchangers. In this article, short time-step analyses of coaxial and double U-tube heat exchangers are performed by means of numerical capacity resistance model. The capacity resistance model is compared with field measurements, analytical solutions available in the literature, and commercial software based on the finite-element method. Further comparisons between coaxial and common double U-tube heat exchangers are carried out by means of measurements performed in a suitable plant system. Furthermore, the series and parallel arrangements for double U-tube heat exchangers are investigated considering the borehole thermal capacitance.

Distributed thermal response tests on a multi-pipe coaxial borehole heat exchanger

In a distributed thermal response test, distributed temperature measurements are taken along a borehole heat exchanger during thermal response tests, allowing the determination of local ground thermal conductivities and borehole thermal resistances. In this article, the first results from six heat injection distributed thermal response tests carried out on a new, thermally insulated leg type, multi-pipe coaxial borehole heat exchanger are presented. The borehole heat exchanger consists of 1 insulated central and 12 peripheral pipes. Temperature measurements are carried out using fiber-optic cables placed inside the borehole heat exchanger pipes. Unique temperature and thermal power profiles along the borehole depth as a function of the flow rate and the total thermal power injected into the borehole are presented. A line source model is used for simulating the borehole heat exchanger thermal response and determining local variations of the ground thermal conductivity and borehole thermal resistance. The flow regime in the peripheral pipes is laminar during all distributed thermal response tests and average thermal resistances remain relatively constant, independently of the volumetric flow rate, being lower than those corresponding to U-pipe borehole heat exchangers. The thermal insulation of the central pipe significantly reduces the thermal shunt to the peripheral pipes even at low volumetric flow rates.[Supplemental Materials are available for this article. Go to the publisher's online edition of HVAC&R Research to view the supplemental file.]

Experimental Research of Thermal Characteristics in Coaxial Radial Heat Pipe

In this paper, heat transfer model is assumed in coaxial radial heat pipe(CRHP) firstly. To verify the model , visualization and performance experiment are studied. Visualization experiments of CRHP are done under different filing radio of 20%, 30%, 35% and inclined angles of 0°(horizontal), 5°, 10°, 15°, 20°. Visualization experimental results show that CRHP of horizontal and 30% filling ratio is better on thermal characteristics. Its heat transfer model is preliminary analyzed from experimental phenomena. In performance experiment , the kind of CRHP is used to test heat transfer characteristics. The temperatures of the outside wall of outer pipe are determined and drawn diagrams. Lastly, heat transfer model is corrected anew by analyzing experimental datas.