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The paper presents a modified in-house model for calculating heat transfer coefficients during flow condensation, which can be applied to a variety of working fluids, but natural refrigerants in particular, at full range thermodynamic parameters with a particular focus on increased saturation pressure. The modified model is based on a strong physical basis, namely the hypothesis of analogy between the heat transfer coefficient and pressure drop in two-phase flow. The model verification is based on a consolidated database that consists of 1286 data points for 7 natural refrigerants and covers the reduced pressure range (the ratio of critical pressure and saturation pressure) from 0.1 to 0.8 for different mass velocities and diameters. The new version of the in-house model, developed earlier by Mikielewicz, was compared with 4 other mathematical models widely recommended for engineering calculations and obtained the best consistency results. The value of the mean absolute percentage error was 28.13% for the modified model, the best result among the scrutinised methods. [ABSTRACT FROM AUTHOR]

The paper presents the results of calculations performed using the authors′ model to predict the heat transfer coefficient during flow boiling for two refrigerants R134a and R1234yf. The experimental data from various past studies have been collected and the calculations have been conducted for the full range of quality variation and a wide range of mass velocity. The aim of the study was to test the sensitivity of the in-house flow boiling and flow condensation model. The importance of taking into account the surface tension as the parameter exhibiting its importance in case of the flow in minichannels, as well as the influence of the reduced pressure were analysed. The obtained numerical results show good consistency with those recorded in the experiments.

Thermal control systems have been introduced as an important part of electronic devices, enabling thermal management of their electronic components. Loop heat pipe (LHP) is a passive two-phase heat transfer device with significant potential for numerous applications, such as aerospace applications, high-power LEDs, and solar central receivers. Its advantages are high heat transfer capability, low thermal resistance, long-distance heat transfer, and compact structure. The essential role of wick structures on the performance of LHPs has already been highlighted, but no comprehensive review is available that deals with different parameters such as LHP design and wick size, which are largely decisive and effective in achieving a practical level of thermal transmission governed by wick structures. To rely on this necessity, this article summarizes, analyzes, and classifies advancements in the design and fabrication of wick structures. The main conclusion to be drawn after careful monitoring and weighing of the related literature is that LHPs with composites and additively manufactured wicks show a higher heat transfer coefficient than other conventional structures. Indeed, future works should be focused on the design of more structurally efficient wicks, which may allow us to optimize materials and geometrical parameters of wick structure for higher heat transfer through LHPs. [ABSTRACT FROM AUTHOR]

In the paper are presented the results using the authors own model to predict heat transfer coefficient during flow boiling. The model has been tested against a large selection of experimental data to investigate the sensitivity of the in-house developed model. In the work are presented the results of calculations obtained using the semi-empirical model on selected experimental flow boiling data of the refrigerants: R134a, R1234yf, R600a, R290, NH3, CO2, R236fa, R245fa, R152a, and HFE7000. In the present study, particular attention was focused on the influence of reduced pressure on the predictions of the theoretical model. The main purpose of this paper is to show the effect of the reduced pressure on the predictions of heat transfer during flow boiling. [ABSTRACT FROM AUTHOR]

In the paper, the original cylindrical heat exchanger with minijets (MJHE) was introduced. The systematic experimental analysis of the prototype heat exchanger was described with special attention paid to such parameters as the heat transfer effectiveness, heat transfer rates, overall heat transfer coefficients, and pressure drop. The heat transfer coefficients were determined based on Wilson plot method, the most suitable approach for heat transfer coefficient determination in exchangers of complex geometry. The thermal-hydraulic characteristics of the in-house manufactured prototype of MJHE in water–water and gas–water configuration are also presented. The experimental results were compared with the predictions from well-known correlations found in the published research papers dedicated to the free-surface and submerged types of jets. [ABSTRACT FROM AUTHOR]

• Flow boiling of R1233zd(E) at reduced pressure ranging from 0.2 to 0.7 considered. • New database for pressure drop and heat transfer for moderate and high reduced pressures. • Flow patterns of R1233zd(E) at moderate and high reduced pressures identified. • Enhanced in-house pressure drop and heat transfer model validated. The results of flow boiling of R1233zd(E) in a 3 mm vertical stainless steel tube are presented at moderate and high saturation temperatures. Integral flow characteristics in the form of pressure drop and heat transfer coefficient are discussed for saturation temperatures ranging from 115 to 145 °C (corresponding reduced pressures from 0.2 to 0.7), mass velocity of 800 kg/m2s and heat flux of 20 kW/m2. All of the obtained heat transfer trends initially decrease with vapor quality and then do not change or increase with vapor quality. The decrease is connected to the dominance of the nucleate boiling phenomenon. The subsequent increase in heat transfer indicates the increased effect of convective boiling. The rate of the increase depends on the value of reduced pressure (the smaller the reduced pressure the higher the rate). The obtained data have been compared with the predictions of the in-house model and some well-known two-phase flow correlations. The in-house model predictions presented the highest accuracy, with MAPE equal to 23.17 % for R1233zd(E) and 19.23 % for R245fa (data from literature). [ABSTRACT FROM AUTHOR]

A model for post dryout mist flow heat transfer is presented based on considerations of energy dissipation in the flow. The model is an extension of authors own model developed earlier for saturated and subcooled flow boiling. In the former version of the model the heat transfer coefficient for the liquid single-phase convection as a reference was used, due to the lack of the appropriate model for heat transfer coefficient for the mist flow boiling. That issue was a fundamental weakness of the former approach. The purpose of present investigation is to fulfil this drawback. Now the reference heat transfer coefficient for the saturated flow boiling is that corresponding to vapour flow the end of the mist flow. The wall heat flux is based on partitioning and constitutes of two principal components, namely the convective heat flux for vapour flowing close to the wall and two phase flow droplet–vapour in the core flowing. Both terms are accordingly modelled. The results of calculations have been compared with some experimental correlations from literature showing a good consistency. [ABSTRACT FROM AUTHOR]

In the paper a model is presented based on energetic considerations for subcooled flow boiling heat transfer. The model is the extension of authors own model developed earlier for saturated flow boiling and condensation. In the former version of the model we used the heat transfer coefficient for the liquid single-phase as a reference level, due to the lack of the appropriate model for heat transfer coefficient for the subcooled flow boiling. That issue was a fundamental weakness of the that approach. The purpose of present investigation is to fulfil this drawback. Now the reference heat transfer coefficient for the saturated flow boiling in terms of the value taking into account the subcooled flow conditions. The wall heat flux is based on partitioning and constitutes of two principal components, namely the convective heat flux and partial evaporation heat flux of the liquid replacing the detached bubble. Both terms are accordingly modeled. The convective heat flux is regarding vapour bubbles travelling longitudinally and the liquid moving radially – liquid pumping. The results of calculations have been compared with some experimental data from literature showing a good consistency. [ABSTRACT FROM AUTHOR]

In the paper presented is analysis of the influence of reduced pressure on efficiency and heat transfer area of vapor generator of Organic Rankine Cycle (ORC) in case of subcritical and supercritical parameters of operation. Compared are two cases of subcritical and supercritical ORC featuring a similar arrangement of heat source supply and heat removal, that is featuring the same temperatures of working fluid before the turbine, and the same condensation temperature in the respective cycles. The analysis is helpful in selection of the appropriate pressure in the vapor generator. In accomplished analyses a selection of wet ORC working fluids are scrutinized for a given range of heat source temperatures with respect to influence on efficiency of thermodynamic cycle and vapor generator area of heat transfer on installation and operation costs to illustrate the issue. Investment cost of a vapor generator in the ORC cycle accounts for a main share of expenditure alongside the cost of the expanding machine. Results of calculations show that from the point of view of cycle efficiency and size of vapor generator the pressures close to critical fluid pressure are usually optimal. Some working fluids feature even an optimal pressure. For the region close to critical point authors elaborated own method for heat transfer coefficients elaboration, which is useful in more exact estimations of heat transfer process in vapor generator. In case of a heat source with a relatively high temperature, it is better to consider a thermodynamic cycle with supercritical parameters even if as a result the vapor generator is slightly larger than for the case of a subcritical cycle. There will always be a more pronounced gain in efficiency compared to the expense induced by the heat transfer surface area of vapor generator. • Effect of reduced pressure on efficiency and heat transfer area of vapor generator is analysed. • An analytical model for determination of heat transfer area of vapor generator is formulated. • Own method for estimating heat transfer coefficient for subcritical and supercritical pressures is applied. • The effect of reduced pressure on efficiency and heat transfer area is shown. [ABSTRACT FROM AUTHOR]

In the paper presented are the results of calculations using authors own model to predict heat transfer coefficient during flow boiling of carbon dioxide. The experimental data from various researches were collected. Calculations were conducted for a full range of quality variation and a wide range of mass velocity. The aim of the study was to test the sensitivity of the in-house model. The results show the importance of taking into account the surface tension as the parameter exhibiting its importance in case of the flow in minichannels as well as the influence of reduced pressure. The calculations were accomplished to test the sensitivity of the heat transfer model with respect to selection of the appropriate two-phase flow multiplier, which is one of the elements of the heat transfer model. For that purpose correlations due to Müller-Steinhagen and Heck as well as the one due to Friedel were considered. Obtained results show a good consistency with experimental results, however the selection of two-phase flow multiplier does not significantly influence the consistency of calculations. [ABSTRACT FROM AUTHOR]

In the paper a method developed earlier by authors is applied to calculations of pressure drop and heat transfer coefficient for flow boiling and also flow condensation for some recent data collected from literature for such fluids as R404a, R600a, R290, R32,R134a, R1234yf and other. The modification of interface shear stresses between flow boiling and flow condensation in annular flow structure are considered through incorporation of the so called blowing parameter. The shear stress between vapor phase and liquid phase is generally a function of nonisothermal effects. The mechanism of modification of shear stresses at the vapor-liquid interface has been presented in detail. In case of annular flow it contributes to thickening and thinning of the liquid film, which corresponds to condensation and boiling respectively. There is also a different influence of heat flux on the modification of shear stress in the bubbly flow structure, where it affects bubble nucleation. In that case the effect of applied heat flux is considered. As a result a modified form of the two-phase flow multiplier is obtained, in which the nonadiabatic effect is clearly pronounced. [ABSTRACT FROM AUTHOR]

• Detailed boiling heat transfer coefficient data for HFE7000 at saturation temperature ranging from 30 to 54 °C. • Experimental procedure for measuring heat transfer coefficient during flow boiling. • Comparison with various heat transfer correlations from literature. • Own model for heat transfer coefficient during flow boiling. This article describes an experimental study of flow boiling of HFE7000 inside a smooth vertical channel. The investigation has been carried out in a circular stainless-steel tube with an inner diameter of 2.3 mm. The data have been collected for the applied heat fluxes q ranging from 61 to 205 kW/m2, the mass flux G ranging from 214 to 1006 kg/(m2 s), the saturation temperature T sat ranging from 30 to 54 °C and the full range of vapour quality x. The collected experimental data base amounted to 1217 experimental points. The acquired results indicated that heat flux and saturation temperature have the most significant impact on the heat transfer coefficient. The local heat transfer coefficient increases with both the heat flux and saturation temperature, while the mass flux did not exhibit a significant effect on the variation of the heat transfer coefficient. The present experimental data have been compared with various heat transfer correlations from literature including the recently enhanced in-house model. The results of comparisons indicated the superiority of the in-house model over other correlations. [ABSTRACT FROM AUTHOR]

In this paper a patented design of a heat exchanger with minijets, with a cylindrical construction is presented. It is followed by the results of its systematic experimental investigations in the single-phase convection heat transfer mode. Based on these results, validation of selected correlations (coming from the literature) describing the Nusselt number was carried out. An assessment of the heat exchange intensification level in the described heat exchanger was done through the comparison with a shell-and-tube exchanger of a classical design. The thermal-hydraulic characteristics of both units were the subjects of comparison. They were constructed for the identical thermal conditions, i.e., volumetric flow rates of the working media and the media temperatures at the inlets to the heat exchanger. The experimental studies of both heat exchangers were conducted on the same test facility. An increase in the heat transfer coefficients values for the minijets heat exchanger was observed in comparison with the reference one, whereas the generated minijets caused greater hydraulic resistance. Experimentally confirmed intensification of heat transfer on the air side, makes the proposed minijets heat exchanger application more attractive, for the waste heat utilization systems from gas sources. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel technique to capture CO 2 including SEC and cyclone separator. • Improving SEC with EHD actuator to overcome resistance and improve convective transfer. • Improving cyclone efficiency with investigation of structural parameters. • Effects of non-condensable gas on hydraulic- thermal parameters of SEC. • Effects of single, dual and quadruple tangential inlets on separation efficiency. The concept for condensation of steam and CO 2 separation in a negative CO 2 emission gas power plant involves the utilization of a steam ejector condenser (SEC) for direct-contact condensation of vapor with inert gas (CO 2) on a spray of subcooled liquid, integrated with a separator to produce pure CO 2. Due to the increasing diffusion resistance and reduced convective heat transfer between the steam and subcooled water phases in the presence of non-condensable gas (CO 2), the study utilized an electrohydrodynamic (EHD) actuator to enhance heat transfer rate in the SEC. To optimize CO 2 purification, the effect of single, dual and quadruple inlets on separation efficiency was analysed. In the SEC, the Eulerian-Eulerian multiphase model is employed, treating water as the continuous phase and the compressible gas mixture (steam and CO 2) as the dispersed phase. The standard k-ε model is chosen to depict the turbulence in the ejector. The separator is transient, turbulent, and three-dimensional, using the control volume method. The RSM turbulent model and mixture model are utilized to simulate the turbulent two-phase flow in the gas–liquid separator. The findings indicated that when the mass flux of steam and voltage are increased, the condensation heat transfer coefficient also increases. For a mass flux of steam of 51 (kg m 2. s) , the condensation heat transfer coefficients were measured to be 0.98, 1.029, 1.08, and 1.134 (MW m 2. K) at electrode voltages of 0, 20, 25, and 30 kV, respectively. In addition, a single-inlet cyclone attains a separation efficiency of 95.1 %, while incorporating two inlets improves the performance to 97.9 %. However, the most remarkable outcome is witnessed in cyclones with four inlets, where an impressive separation efficiency of 99.9 % is achieved. [ABSTRACT FROM AUTHOR]

• New database for flow boiling of R1233zd(E) for moderate and high reduced pressures. • New database for pressure drop of R1233zd(E) for moderate and high reduced pressures. • Reduced pressure considered in the experiment ranged from 0.2 to 0.7. • Enhanced in-house heat transfer model. The paper presents the results of experimental study of flow boiling of R1233zd(E) in minichannel at moderate and high saturation temperatures. The heat transfer coefficient and pressure drop were measured for flow boiling in a stainless steel tube with 2 mm inside diameter and a length of 300 mm. Experiment has been conducted for saturation temperatures ranging from 83 to 145 °C which corresponds to values of reduced pressures ranging from 0.2 to 0.7. Three different heat fluxes were analyzed, namely 20, 30 and 45 kW/m2 and mass velocities ranged from 200 to 1000 kg/m2s. The effects of reduced pressure, mass velocity, heat flux, vapor quality and flow regime were analyzed. At low reduced pressures (p r ≤ 0.3) heat transfer is driven by convective boiling mostly with nucleate boiling having effect only in the case of low mass velocities. Heat transfer in this case increases with mass velocity and vapor quality and is independent of heat flux. For reduced pressures of 0.4 and 0.5 both nucleate and convective boiling are present, however the nucleate boiling starts to be more dominant here, especially for low mass velocities and high heat fluxes. For low mass velocities and/or higher heat fluxes the heat transfer decrease or is independent of vapor quality, whereas for higher mass fluxes and/or lower heat fluxes it is initially independent of vapor quality and starts to increase close to the transition from intermittent to annular flow. At the range of high reduced pressures (p r ≥ 0.6) nucleate boiling is the dominant heat transfer mechanism. Experimental results were compared with some well-known two-phase flow correlations. [ABSTRACT FROM AUTHOR]