Experimental investigation of a two-phase closed thermosyphon solar water heater

In this study, experiments were performed to find out how the thermal performance of a two-phase thermosyphon solar collector was affected by using different refrigerants. Three identical small-scale solar water heating systems, using refrigerants R-134a, R407C, and R410A, were constructed and tested side-by-side under various environmental and load conditions. The performance of the system under clear-sky conditions has been investigated with and without water load. Detailed temperature distributions and cumulative collection efficiencies were determined and presented. The experimental results were compared to the results found in the literature and they showed good agreement.     

Numerical investigations and engineering applications on freezing expansion of soil restrained two-phase closed thermosyphons

A numerical simulation has been carried out to describe the coupled heat transfer of water-saturated soil with a two-phase closed thermosyphon. This problem is characterized by the phase change occurring in the water-saturated soil, moving freezing front and heat transfer of a two-phase closed thermosyphon. According to the governing equations in the porous media and heat transfer characteristics of the two-phase closed thermosyphon, the temperature distributions in the water-saturated soil and the moving freezing front are solved numerically by the finite-difference method. The predictions of the present study are well agreed with the measured data. The mechanism of the freezing expansion restrained by the two-phase closed thermosyphon is exposed, based on which the effective radius can be determined for engineering applications.     

Development of a thermoelectric refrigerator with two-phase thermosyphons and capillary lift

A thermoelectric domestic refrigerator has been developed, with a single compartment of 0.225 m³, for food preservation at 5 °C. The cooling system is made up of two equal thermoelectric devices, each composed of a Peltier module (50 W) with its hot side in contact with a two-phase and natural convection thermosyphon (TSV) and a two-phase and capillary lift thermosyphon (TPM), in contact with the cold side.The entire refrigerator has been simulated and designed using a computational model, based on the finite difference method. Subsequently an experimental optimization phase of the thermosyphons was carried out, until thermal resistance values of R_TSV = 0.256 K/W and R_TPM = 0.323 K/W were obtained. These values were lower than those obtained with finned heat sinks.Finally, a functional prototype of a thermoelectric refrigerator was built, and the results which were obtained demonstrate that it is able to maintain a thermal drop (Ambient Temperature–Inside Temperature) of 19 °C. The electric power consumption at nominal conditions was 45 W, reaching a COP value of 0.45. The study demonstrated that by incorporating these two-phase devices into thermoelectric refrigeration increases the COP by 66%, compared with those which use finned heat sinks.     

Experimental investigation of two-phase closed thermosyphon used in heavy duty extrusion pelleting line

Efficient cooling system is an essential part of heavy duty extrusion pelleting line which plays an important role in production engineering of megaton polyolefin. In this paper, a new cooling system based on two-phase closed thermosyphon (TPCT) used in heavy duty extrusion pelleting line was presented. Comparative experimental results show that thermal performance of TPCT is more efficient, and the temperature uniformity is much better than traditional extruder (similar to coiled heat exchanger) in preheating process and extrusion reaction process. The effects of different operation conditions: filling ratio (0.2, 0.35, 0.5, 0.65 and 0.8), flow rate of cooling water (120 L/h, 180 L/h, 240 L/h, 300 L/h and 360 L/h) and heating power (7 kW, 9 kW and 11 kW) on thermal characteristics were experimental investigated, respectively. The results show that temperature of barrel inner wall increased significantly as filling ratio increased and evaporator appeared dry out when filling ratio is 0.2. The flow rate of cooling water affected the condenser section obviously, but had little influence on evaporation section. With increase of heating power, the start-up time decreases and the heat transfer coefficient increased. An ideal cooling scheme was concluded: the liquid filling ratio was 0.35, the cooling water flow rate was 180 L/h and the heating power was 11 kW when working medium was water.     

小直径水滴惯性分离阻力特性的试验研究

针对湿法脱硫、烟气余热利用、湿法除尘等工业实践中出现的气流携带小直径水滴的分离进行了 试验研究。在分析气流携带水滴机理的基础上,对各个因素对分离阻力的影响进行了实验。实验结果 表明,含有小直径水滴的气体通过分离件时所产生的阻力较小,最大值不超过70Pa。在保证分离效率 的前提下,槽形和角形分离件具有流动阻力小的特点。对正交实验结果进行了直观分析,正交实验结果 和理论分析相吻合。表实验研究结果对于小直径水滴分离设备的工程设计具有一定的指导意义。     

Experimental investigation of a thermoelectric refrigeration system employing a phase change material integrated with thermal diode (thermosyphons)

This paper presents results of tests carried out to investigate the potential application of phase change materials (PCMs) integrated with thermosyphons in a thermoelectric refrigeration system. The work involved design, fabrication and test of a 150 W thermoelectric refrigeration system. The system was first fabricated and tested using a conventional heat sink system (bonded fin heat sink system) at the cold heat sink. In order to improve the performance and the storage capability, the system was reconstructed and tested using an encapsulated PCM as a cold sink. Results of tests of the latter system showed an improved performance compared with the former system. However to improve the storage capability, in particular during off-power periods, it was found necessary to integrate the PCM with a thermal diode, which would allow heat flow in one direction only. Results of tests carried out on the new system showed considerable improvement in the storage capability of the thermoelectric refrigeration system compared with the previous ones. Overall the system suits operation with renewable energies, e.g., solar energy.     

An experimental and theoretical investigation of the transient behavior of a two-phase closed thermosyphon

This paper presents an experimental and theoretical investigation of the two-phase closed thermosyphon (TPCT) behavior in transient regimes. Experimental results show two kinds of TPCT response. We focus on regular variations of operating system variables, where a mathematical model has been developed in order to obtain an analytical expression of the system response time. The dependence of this response time according to the various parameters is linked to geometry and heat transfer laws. The model can be considered as a simple and efficient tool for designing TPCTs in both transient and steady regimes.     

Experimental investigation of the loop thermosyphon with different adiabatic lengths charged with different working fluids

This paper reports an experimental investigation of a closed-loop thermosyphon system charged with water and other low saturation fluids, such as ethanol, acetone, and methanol, for different adiabatic lengths, filling ratios, and heat loads. The closed-loop thermosyphon with two inline vertical heaters in the evaporator section and forced air-cooled plate-type heat exchanger in the condenser section, connected by a changeable adiabatic length, is investigated at different working conditions. Out of five filling ratios used in the analysis, at 0.6 filling ratio, the loop thermosyphon is seen to be operated at its best. The acetone-charged loop thermosyphon shows the lowest values (up to 72% reduction) of overall thermal resistance than that of other fluids and significantly higher effectiveness, due to the plate-type forced air-cooled condenser. For the acetone-filled thermosyphon, an almost 15% increase in the effectiveness is observed by changing the adiabatic length from 800 to 200 mm. This study suggests that the limitation of the loop thermosyphon with a water-cooled condenser to cool electronic components, computational clusters, and data centers is well fulfilled by the loop thermosyphon with plate-type forced air-cooled condenser. The nucleate pool boiling correlation is developed and validated for the loop thermosyphon system to determine the evaporator heat transfer coefficient.     

Boiling of water and FC-72 in microchannels enhanced with novel features

A microchannel test section comprised of parallel square microchannels with a 25 × 25 μm and 50 × 50 μm cross section was manufactured. Boiling of perfluorinated dielectric fluid FC-72 and water in microchannels was studied. Troublesome occurrences associated with flow boiling in microchannels were reduced or eliminated with inlet/outlet restrictors, inlet/outlet manifolds and potential nucleation cavities incorporated in the array of microchannels. The gradual reduction of channel cross section in the manifolds ensured a uniform distribution of the working fluid among the microchannels. The flow restrictors provided a higher upstream pressure drop in comparison with the downstream pressure drop which favors vapor flow in the downstream direction and consequentially suppresses the vapor backflow present in flow boiling. The superheat of the microchannel wall necessary for the onset of boiling was decreased significantly with the incorporation of properly sized artificial cavities. Experimental results confirmed the benefits of the etched features, as there was (i) an even working fluid distribution (ii) without dominating backflows of vapor (iii) at a low temperature of the onset of boiling. Bubble growths as well as other events in the microchannels were visualized with a high-speed imaging system which captured images at over 87,000 frames per second. Results exhibit boiling hysteresis dependence of the working fluid and its mass flux through the microchannels. The temperature of the onset of boiling is highly dependent on the working fluid, microchannel size and its roughness.     

Experimental investigation on two-phase flow boiling heat transfer of five refrigerants in horizontal small tubes of 0.5, 1.5 and 3.0 mm inner diameters

An experimental investigation on two-phase flow boiling heat transfer with refrigerants of R-22, R-134a, R-410A, C₃H₈ and CO₂ in horizontal circular small tubes is presented. The experimental data were obtained over a heat flux range of 5–40 kW m^?2, mass flux range of 50–600 kg m^?2 s^?1, saturation temperature range of 0–15 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 0.5, 1.5 and 3.0 mm, and lengths of 330, 1000, 1500, 2000 and 3000 mm. The experimental data were mapped on Wang et al. (1997) [5] and Wojtan et al. (2005) [6] flow pattern maps. The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the heat transfer coefficient are reported. The experimental heat transfer coefficients were compared with some existing correlations. A new boiling heat transfer coefficient correlation that is based on a superposition model for refrigerants in small tubes is presented with 15.28% mean deviation and ?0.48% average deviation.     

Experimental and numerical investigation of premixed flame propagation with distorted tulip shape in a closed duct

High-speed schlieren photography, pressure records and large eddy simulation (LES) model are used to study the shape changes, dynamics of premixed flame propagation and pressure build up in a closed duct. The study provides further understanding of the interaction between flame front, pressure wave and combustion-generated flow, especially when the flame acquires a “distorted tulip” shape. The Ulster multi-phenomena LES premixed combustion model is applied to gain an insight into the phenomenon of “distorted tulip” flame and explain the experimental observations. The model accounts for the effects of flow turbulence, turbulence generated by flame front itself, selective diffusion, and transient pressure and temperature on the turbulent burning velocity. The schlieren images show that the flame exhibits a salient “distorted tulip” shape with two secondary cusps superimposed onto the two original tulip lips. This curious flame shape appears after a well-pronounced classical tulip flame is formed. The dynamics of “distorted tulip” flame observed in the experiment is well reproduced by LES. The numerical simulations show that large-scale vortices are generated in the burnt gas after the formation of a classical tulip flame. The vortices remain in the proximity of the flame front and modify the flow field around the flame front. As a result, the flame front in the original cusp and near the sidewalls propagates faster than that close to the centre of the original tulip lips. The discrepancy in the flame propagation rate finally leads to the formation of the “distorted tulip” flame. The LES model validated previously against large-scale hydrogen/air deflagrations is successfully applied in this study to reproduce the dynamics of flame propagation and pressure build up in the small-scale duct. It is confirmed that grid resolution has an influence to a certain extent on the simulated combustion dynamics after the flame inversion.     

Experimental investigation and numerical modelling of transient two-phase flow in a geysering geothermal well

Measurements of the periodic transient flow in a vertical geysering geothermal well are presented. The 70-m deep, 0.1-m internal diameter well taps a hot (about 87 °C) water aquifer rich in dissolved carbon dioxide. Transient pressures measured at various depths show the various flow regimes that develop in the borehole and demonstrate that the periodic flow is caused by the degassing of the water flowing up the well. A one-dimensional numerical model of the flow has been developed. The computed results exhibit the main characteristics of the test measurements. This agreement between model and measurements is considered to support both the numerical model and the conceptual model of the system deduced from the measurements.     

Experimental investigation of start-up and transient thermal performance of pumped two-phase battery thermal management system

In this study, a pumped two-phase battery thermal management system was developed, and its start-up and transient thermal performances were experimentally evaluated. The start-up behavior was characterized, and the effects of the flow rate, heat flux, and cold-source temperature on the start-up and transient thermal performances were examined. Three start-up modes were observed: fluctuating growth, temperature overshoot, and smooth growth. The fluctuating growth start-up mode appears to be suitable for battery cooling. The transient performance was improved when the flow rate was decreased, which resulted in a quicker start-up and lower average temperature (t_avg) and maximum temperature difference (∆t_max). Reducing the flow rate from 0.99 to 0.20 L/min significantly shortened the start-up time, lowered t_avg and ∆t_max, and increased the heat transfer coefficient (α) when the steady state was reached. Increasing the heat flux initially improved and then weakened the transient performance of the pumped two-phase system. Increasing the heat flux from 1.1 to 2.8 W/cm² initially reduced the start-up time and t_avg to 350 seconds and 1.5°C, respectively, but they then significantly increased to 360 seconds and 13.5°C, respectively. The transient t_avg and ∆t_max decreased with the cold-source temperature (t_cs), while the start-up time was independent of changes in t_cs.     

Experimental and numerical investigation of two-phase pressure drop in vertical cross-flow over a horizontal tube bundle

Experimental pressure drop data for vertical two-phase air–water flow across horizontal tubes is presented for gas mass fractions in the range 0.0005–0.6 and mass fluxes in the range 25–700 kg/m² s. The square in-line tube bundle had one column containing ten tubes and two columns of half tubes attached to the walls. The tubes had a diameter of 38 mm and a pitch to diameter ratio of 1.32. This data and air–water and R113 vapour–liquid data available in the literature are compared with the predictions from two kettle reboiler models, the one-dimensional model and a one-dimensional formulation of the two-fluid model. The one-dimensional model was implemented with three separate void fraction correlations and one two-phase friction multiplier correlation. The results show that the two-fluid model predicts air–water void fraction data well but R113 data poorly with pressure drop predictions for both being unsatisfactory. The one-dimensional model is shown to predict pressure drop and void fraction data reasonably well, provided a careful choice is made for the void fraction correlation.     

Experimental investigation of solar assisted hydrogen production from water and aluminum

Sustainable production of hydrogen at high capacities and low costs is one the main challenges of hydrogen as a future alternative fuel. In this paper, a new hydrogen production system is designed and fabricated to investigate hydrogen production using aluminum and solar energy. Numerous experiments are performed to evaluate the hydrogen production rate, quantitatively and qualitatively. Moreover, correlations between the total hydrogen production volume over time and other parameters are developed and the energy efficiency and conversion ratio of the system are determined. Also, a method is developed to obtain an optimal and stable hydrogen production rate based on system scale and consumed materials. It is observed that at low temperatures, the hydrogen production volume, efficiency and COP of the system increase at a higher sodium hydroxide molarity. In contrast, at high temperatures the results are vice versa. The maximum hydrogen production volume, hydrogen production rate, reactor COP and system efficiency using 0.5 M NaOH solution containing 3.33 g lit^?1 aluminum at 30 °C are 6119 mL, 420 mL min^?1, 1261 mL H₂ per 1 g of Al, and 16%, respectively.     

Flow boiling heat transfer in two-phase micro-channel heat sinks--I. Experimental investigation and assessment of correlation methods

This paper is the first of a two-part study concerning measurement and prediction of saturated flow boiling heat transfer in a water-cooled micro-channel heat sink. In this paper, new experimental results are discussed which provide new physical insight into the unique nature of flow boiling in narrow rectangular micro-channels. The micro-channel heat sink contained 21 parallel channels having a m cross-section. Tests were performed with deionized water over a mass velocity range of 135-402 kg/m² s, inlet temperatures of 30 and 60 °C, and an outlet pressure of 1.17 bar. Results indicate an abrupt transition to annular flow near the point of zero thermodynamic equilibrium quality, and reveal the dominant heat transfer mechanism is forced convective boiling corresponding to annular flow. Contrary to macro-channel trends, the heat transfer coefficient is shown to decrease with increasing thermodynamic equilibrium quality. This unique trend is attributed to appreciable droplet entrainment at the onset of annular flow regime development, and the increase in mass flow rate of the annular film by droplet deposition downstream. Eleven previous empirical correlations are assessed and deemed unable to predict the correct trend of heat transfer coefficient with quality because of the unique nature of flow boiling in micro-channels, and the operating conditions of water-cooled micro-channel heat sinks falling outside the recommended application range for most correlations. Part II of this study will introduce a new annular flow model as an alternative approach to heat transfer coefficient prediction for micro-channels.     

Experimental investigation of moderately high temperature water source heat pump with non-azeotropic refrigerant mixtures

Experimental investigations were carried out on non-azeotropic refrigerant mixtures, named M1A (mass fraction of 20%R152a and 80%R245fa), M1B (mass fraction of 37% R152a and 63%R245fa) and M1C (mass fraction of 50%R152a and 50%R245fa), based on a water-to-water heat pump system in the condensing temperature range of 70–90 °C with a cycle temperature lift of 45 °C. Performance of R245fa was tested for comparison. Unfair factors in experimental comparative evaluation research with the same apparatus were identified and corrected. Experimental cycle performance of the mixtures were tested and compared with improved experimental assessment methodology. The results show that all of the mixtures deliver higher discharge temperature, higher heating capacity, higher COP and higher ε_h,c than R245fa. M1B presents the most excellent cycle performance and is recommended as working fluid for moderate/high temperature heat pump.     

An experimental investigation of critical flow rates of subcooled water through short pipes with small diameters

Critical two-phase flow rates of subcooled water through short pipes (L < 400 mm) with small diameters (D < 7.15 mm) have been experimentally investigated for wide ranges of subcooling (0 ∼ 199 °C) and pressure (0.5 ∼ 2.0 MPa). To examine the effects of various parameters (i.e., the location of flashing inception, the degree of subcooling, the stagnation temperature and pressure, and the pipe size) on the critical two-phase flow rates of subcooled water through short pipes with small diameters, a total of 135 runs were made for various combinations of test parameters using four different test sections. Experimental results that show effects of various parameters on subcooled critical two-phase flow rates are presented in the form of graphs such as the dimensionless mass flux () versus the dimensionless subcooling () curve. An empirical correlation expressed in terms of a dimensionless subcooling () is also obtained for subcooled two-phase flow rates through present test sections. Comparisons between the mass fluxes calculated by present correlation and a total of 679 selected experimental data points of 9 different investigators show that the agreement is fairly good except for very low subcooling data obtained from small (less than 10) orifices.