Superheated Vapor Region Research Articles

Analysis of complete boiling process inside double pipe porous heat exchanger filled with NanoFluids

This study involves an investigation on fully evaporation process inside annular porous heat exchanger using Cu/distilled water nano-fluid. Emphasis has been placed on how the use of Cu/distilled water affected two-stage flux predictions. Two-phase mixture model, based on the considerations of Local Thermal Equilibrium (LTE), has been reformulated to involve the influences of employing nano-fluid instead of only water. To display the actions of employing nano-fluid, the predictions of liquid saturation and temperature, obtained using Cu/distilled water and only water, have been presented and compared for a wide ranges of geometric conditions, operating conditions and properties of porous media. The outcomes displayed that the using Cu particle to water has a large impacts on the heat diffusivity, where the boiling and condensation fronts are considerably moved to downstream and upstream sections, respectively, as compared to the predictions of only water due to rising the axial diffusion. The temperature at the pipe exit is considerably decreased when adding Cu particles to water as compared to the pure water case. The predications of Cu/distilled water show that the nano-fluid volume fraction, geometric parameters, operating parameters and porous structure properties have high actions on the locations of initiation and termination of phase change process. Since the fluid temperature for pure water case is extremely high at the superheated vapour region, the addition of Cu particles to water considerably reduces the fluid temperature in this region and hence, the use of nano-fluid can be considered as a safety factor in the applications that used heat exchangers (boiling processes) due to it improves the mechanism of heat transfer during the boiling process.

Experimental Study of Horizontal Flow Boiling Heat Transfer Coefficient and Pressure Drop of R134a from Subcooled Liquid Region to Superheated Vapor Region

For the past few years, research in the field of flow boiling heat transfer has gained immense popularity for unravelling the dominant mechanism responsible for controlling heat transfer and identifying a parametric trend for understanding the characteristics of flow boiling heat transfer. This has led to several assumptions and models for predicting heat transfer during flow boiling without any known generalized mechanism. This study therefore seeks to experimentally study the characteristics of heat transfer during flow boiling over a wide range but small increase in vapor quality from a single-phase subcooled region through to a two-phase superheated vapor region. The study was performed with an R134a refrigerant in a single horizontal circular stainless-steel smooth tube that had an internal diameter of 5 mm. In this experimental study, local heat transfer coefficients and frictional pressure drop were measured for low heat fluxes of 4.6–8.5 kW/m2, mass fluxes of 200–300 kg/(m2s), vapor quality from −0.1 to 1.2 and a low constant saturation pressure of 460 kPa. Flow patterns observed during the study were recorded with a high-speed camera at 2000 fps. In covering a wide range of vapor quality, a peak of heat transfer coefficient near a vapor quality of zero and a local minimum observed in the low vapor quality region were observed, and both were sensitive to heat flux and mildly sensitive to mass flux. Generally, at low vapor quality, the heat transfer coefficient deteriorated with vapor quality and this was sensitive to heat flux but insensitive to mass flux and vapor quality, indicating nucleate boiling dominance in low vapor quality regions. In high vapor quality regions, the heat transfer coefficient was sensitive to mass flux and insensitive to heat flux. This indicates the dominance of convective boiling. In the low vapor quality regions, the flow patterns observed were slug and intermittent, while in the high vapor quality region, annular and mist flow patterns were observed. Generally, frictional pressure drop increased with increasing mass flux and vapor quality in the two-phase region.

Artificial Bee Colony-Artificial Neural Network (ABC-ANN) Hybrid Algorithm’s Performance on the Modeling of Thermodynamic Properties of a Refrigerant Gas (R404a)

ABSTRACT In this study, it was aimed to determine the thermodynamic properties of an environmentally friendly refrigerant (R404a) for both the saturated liquid–vapor region (wet vapor) and superheated vapor region, in the temperature range of 173–498°K, and the pressure range of 10–3600 kPa by using an adaptive artificial neural network algorithm. Performing the analysis of these gases with differential equations by using a computer is very time consuming and requires high computational power for calculation. Using numerical equations for modeling the thermodynamic properties of gasses to eliminate these drawbacks is a more accurate approach and this modeling can be accomplished with artificial intelligence algorithms such as the Artificial Neural Network (ANN). The proper selection of the activation function, which is one of the most important parameters for ANN, directly affects the validity of the model according to the problem and its application. In this study, an adaptive ANN was developed in which the optimal activation function combination was found by using the ABC algorithm and thus the error of the network were reduced when compared to the classical ANN. The improvements of in percentage errors were observed to increase from 7.55% to 76.68%. Finally, the accuracy of the numerical equations that describe the thermodynamic properties of R404a gas was increased. Using this technique helps to figure out the performance of the gas under related working conditions.

Analysis on gas dynamic effects and design of supersonic ORC stator nozzles for transcritical expansions

Organic Rankine Cycles can reach high efficiency by using transcritical cycles, where the fluid expansion starts in the supercritical region and ends in the superheated vapor region. When molecularly complex fluids are used, their gas dynamic behavior differs significantly from that of expanding ideal gases, and their low speed of sound results in supersonic conditions even with moderate flow velocities. In this study, gas dynamic effects and supersonic nozzle designs for turbines are investigated. Isentropic expansions in conditions near the critical point are considered for different working fluids at varying nozzle inlet total conditions. The reduced total pressure is varied in the range 0.8 to 1.5 and the reduced total temperature in the range 0.95 to 1.20. The fluid and operating conditions have significant effect on the gas dynamic behavior and characteristics of the expansion, as well as on the resulting size of supersonic nozzles. The most significant non-ideal behavior was observed with siloxanes MDM and MM, and hydrocarbons toluene and cyclohexane. The variations in thermodynamic properties along the expansions were observed to be significantly large, especially if the expansions start from the supercritical region. Correspondingly, remarkable variations in the required passage areas and nozzle length were observed.

Identification of the General Anesthesia Induced Loss of Consciousness by EEG Spectrum Analysis

Organic Rankine Cycles can reach high efficiency by using transcritical cycles, where the fluid expansion starts in the supercritical region and ends in the superheated vapor region. When molecularly complex fluids are used, their gas dynamic behavior differs significantly from that of expanding ideal gases, and their low speed of sound results in supersonic conditions even with moderate flow velocities. In this study, gas dynamic effects and supersonic nozzle designs for turbines are investigated. Isentropic expansions in conditions near the critical point are considered for different working fluids at varying nozzle inlet total conditions. The reduced total pressure is varied in the range 0.8 to 1.5 and the reduced total temperature in the range 0.95 to 1.20. The fluid and operating conditions have significant effect on the gas dynamic behavior and characteristics of the expansion, as well as on the resulting size of supersonic nozzles. The most significant non-ideal behavior was observed with siloxanes MDM and MM, and hydrocarbons toluene and cyclohexane. The variations in thermodynamic properties along the expansions were observed to be significantly large, especially if the expansions start from the supercritical region. Correspondingly, remarkable variations in the required passage areas and nozzle length were observed.

Development of a thermal-hydraulic analysis code for thermal sizing of once-through steam generators for SMR

A one-dimensional thermal-hydraulic analysis code was developed for the conceptual design and sizing of once-through steam generators (OTSGs) with application to small modular nuclear reactors. The OTSG in the model is represented by one characteristic double-tube heat exchanger with countercurrent flow. The secondary side of the OTSG is divided into subcooled liquid region, saturated two-phase mixture region, and superheated vapor region. Heat transfer and pressure drop correlations for each flow region were selected and implemented. To benchmark predictability of the developed computer code, the B&W OTSG whose design data is available in open literature was simulated. The results were compared both to the design data of the OTSG and to the analysis results obtained using a best-estimate thermal-hydraulic analysis code. The overall characteristics of the heat transfer area, pressure, and temperature distributions calculated with the developed code were in good agreement with the design data as well as the results from the best-estimate code. Lastly, a new concept of OTSG that incorporates feedwater circulation to reduce its volume for SMR application was simulated using the developed code. This demonstrates that the code can be used for design innovation and performance analysis of OTSGs.

Two-Phase and Vapor-Phase Thermophysical Property (pvTz) Measurements of the Difluoromethane + trans-1,3,3,3-Tetrafluoroprop-1-ene Binary System

In this paper, 182 pvTz data (28 data in the two-phase region and 154 data in the superheated vapor region) of mixtures containing difluoromethane (R32) and trans-1,3,3,3-tetrafluoroprop-1-ene (R12...

Numerical investigation of two-phase flow in a horizontal porous evaporator with localised heating using non-Darcian flow and two equations model

In this article, a new mathematical model, based on the modified enthalpy formulation of Two-Phase Mixture Model (TPMM), is developed by considering non-Darcy flow and Local Thermal Non-Equilibrium (LTNE) conditions to describe numerically the two-dimensional problems of the complete evaporation process inside an asymmetrically heated porous evaporator. The governing equations have been discretised using Finite Volume Method (FVM) on a fixed non-staggered grid layout. The in-house code has been validated against experimental data and the results show good agreement between them. Three different models have been employed for the partitioning of the wall heat flux. Only the effect of the heat flux has been analysed, while all other parameters and properties have been kept fixed during the present investigation. The numerical simulations indicated that the locally high heat flux significantly influences the working fluid flow performances, and lead to extend the superheated vapour region towards the exit of the evaporator. The results also show that the two-phase zone significantly expanded in the axial direction and does not occupy a large portion inside the channel due to the energy transport through the two-phase region is significantly higher in the flow direction as compared to the transverse direction, caused by the decrease in the effective diffusion coefficient in this region and higher axial velocity. Numerical analysis reveal that the different models for the partitioning of the wall heat flux have almost no influence on the overall predictions, except in the vicinity of the wall. The results clearly demonstrate that the numerical simulations with the modified h-formulation by considering non-Darcy law and LTNE model are indispensable for the problem of complete evaporation process, particularly under the high both heat flux and inlet velocity conditions.

Vapor phase and two-phase PvTz measurements of difluoromethane + 2,3,3,3-tetrafluoroprop-1-ene

This paper provides 217 PvTz points for the binary systems of difluoromethane (R32) and 2,3,3,3-tetrafluoroprop-1-ene (R1234yf) both in the two-phase and superheated vapor regions. The R32 + R1234yf data were measured along eleven isochores (0.029794, 0.029936, 0.030724, 0.032113, 0.061323, 0.063915, 0.102150, 0.111536, 0.116592, 0.144796, 0.280627) m3·kg−1 for temperatures from (263 to 383) K for eleven R32 mole fractions (0.1214, 0.1330, 0.1792, 0.2712, 0.3432, 0.5229, 0.6231, 0.6489, 0.7020, 0.8122, 0.9460). From the measurements in the two-phase region, the vapor-liquid equilibrium behavior of the studied binary systems was derived by applying the flash method with the Carnahan–Starling–De Santis equation of state, the Peng–Robinson equation of state and a two-parameter cubic equation of state proposed by Stryjek. The representation of the vapor – liquid equilibrium for the studied binary systems obtained from the isochoric data is consistent with the experimental vapor – liquid equilibrium data available in the literature. The superheated vapor data were correlated using the virial equation of state, in addition to the aforementioned equations of state. The vapor phase data showed good agreement with the values calculated with the equations of state and the predictions provided by REFPROP 10.0.

Thermodynamic Selection of the Optimal Working Fluid for Organic Rankine Cycles

A novel method proposed to choose the optimal working fluid—solely from the point of view of expansion route—for a given heat source and heat sink (characterized by a maximum and minimum temperature). The basis of this method is the novel classification of working fluids using the sequences of their characteristic points on temperature-entropy space. The most suitable existing working fluid can be selected, where an ideal adiabatic (isentropic) expansion step between a given upper and lower temperature is possible in a way, that the initial and final states are both saturated vapour states and the ideal (isentropic) expansion line runs in the superheated (dry) vapour region all along the expansion. Problems related to the presence of droplets or superheated dry steam in the final expansion state can be avoided or minimized by using the working fluid chosen with this method. Results obtained with real materials are compared with those gained with model (van der Waals) fluids; based on the results obtained with model fluids, erroneous experimental data-sets can be pinpointed. Since most of the known working fluids have optimal expansion routes at low temperatures, presently the method is most suitable to choose working fluids for cryogenic cycles, applied for example for heat recovery during LNG-regasification. Some of the materials, however, can be applied in ranges located at relatively higher temperatures, therefore the method can also be applied in some limited manner for the utilization of other low temperature heat sources (like geothermal or waste heat) as well.

Analysis of variable porosity, thermal dispersion, and local thermal non-equilibrium on two-phase flow inside porous media

This work involves the numerical simulations of two-phase flow inside porous media and the associated phase change processes in order to investigate the effects of thermal dispersion and variable porosity near the wall boundary. The modified enthalpy formulation has been employed. The mathematical model for the conservation of energy is based on the assumption of Local Thermal Non-Equilibrium (LTNE) condition. The relevant parameters such as free stream porosity, pore diameter, thermal conductivity of the solid phase, heat flux and Reynolds number have been used to analyse the significance of the above mentioned effects. The governing equations have been discretised using the Finite Volume Method (FVM). The developed two-dimensional code has been validated against experimental data, which definitely display that there is a good agreement between them. The numerical results indicated that considering the effects of thermal dispersion and variable porosity is significant, particularly near the heated wall. It is also observed that those two effects become more pronounced for the large values of pore diameter, free stream porosity and Reynolds number. The effect of variations in the free stream porosity, Reynolds number and the pore diameter becomes substantial near the wall when thermal dispersion and variable porosity effects are included, whereas the effect of variations in the heat flux and thermal conductivity of the solid phase have minor influence on the temperature distribution near the heated wall. It is evident that adding the effect of variable porosity and thermal dispersion have significant impact on the initiation and termination of phase change process, especially in the superheated vapour region as compared to the results that obtained by excluding those two effects. In general, it has been found that the present model provides a lower maximum fluid temperature near the wall as compared to that obtained by excluding those effects. Therefore, excluding these effects may lead to inaccurate predictions of two-phase flow with phase change process inside porous media. Most importantly, the thermal dispersion in the transverse direction plays an important role in the dispersion phenomenon inside porous media owing to the thermal boundary layer growth is more dependent on the transverse diffusion as compared to the axial diffusion.

Two-phase explicit CBS procedure for compressible viscous flow transport in porous materials

PurposeIn this work, a new two-phase version of the finite element-based Artificial Compressibility (AC) Characteristic-Based Split (CBS) algorithm is developed and applied for the first time to heat and mass transfer phenomena in porous media with associated phase change. The purpose of this study is to provide an alternative for the theoretical analysis and numerical simulation of multiphase transport phenomena in porous media. Traditionally, the more complex Separate Flow Model was used in which the vapour and liquid phases were considered as distinct fluids and mathematically described by the conservation laws for each phase separately, resulting in a large number of governing equations.Design/methodology/approachEven though the adopted mathematical model presents analogies with the conventional multicomponent mixture flow model, it is characterized by a considerable reduction in the number of the differential equations for the primary variables. The fixed-grid numerical formulation can be applied to the resolution of general problems that may simultaneously include a superheated vapour region, a two-phase zone and a sub-cooled liquid region in a single physical domain with irregular and moving phase interfaces in between. The local thermal non-equilibrium model is introduced to consider the heat exchange between fluid and solid within the porous matrix.FindingsThe numerical model is verified considering the transport phenomena in a homogenous and isotropic porous medium in which water is injected from one side and heated from the other side, where it leaves the computational domain in a superheated vapour state. Dominant forces are represented by capillary interactions and two-phase heat conduction. The obtained results have been compared with the numerical data available in the scientific literature.Social implicationsThe present algorithm provides a powerful routine tool for the numerical modelling of complex two-phase transport processes in porous media.Originality/valueFor the first time, the stabilized AC-CBS scheme is applied to the resolution of compressible viscous flow transport in porous materials with associated phase change. A properly stabilized matrix inversion-free procedure employs an adaptive local time step that allows acceleration of the solution process even in the presence of large source terms and low diffusion coefficients values (near the phase change point).

Modelling and investigation on heat transfer deterioration during transpiration cooling with liquid coolant phase-change

In this paper, a new mathematical model is suggested to describe the two-dimensional problems of the transpiration cooling with liquid coolant phase change, to simulate and analyze the effects of locally high heat flux on surface temperature and coolant flow within porous matrix. The numerical simulations indicate that the locally high heat flux significantly influences the coolant flow performances, and lead to distinct heat transfer deterioration, i.e. local vapor blocks the porous matrix, and liquid coolant flows into the region far from the superheated vapor region, which results in a higher temperature of the superheated region. Numerical analyses of different parameters reveal that the heat transfer deterioration is more serious in uniform porous matrix with a lower thermal conductivity and a larger porosity. To prevent the heat transfer deterioration, this paper demonstrates and compares the following three approaches: (1) varying local porosity and conductivity of the porous matrix; (2) using two solid walls to separate the porous matrix into three chambers; (3) varying the local thickness of the porous matrix. Using the three approaches, the maximal temperature at hot side can drop by approximately 40%, 55% and 5%, respectively. The comparisons show that the second method is more effective.

Numerical analysis on thermal characteristics of transpiration cooling with coolant phase change

A numerical investigation of phase change transpiration cooling has been conducted in this work using the modified separate flow model, in which the effects of capillarity, non-isothermal characteristics in two-phase region and the local thermal non-equilibrium characteristics between the coolant and the matrix have been considered to describe the liquid coolant phase change and heat exchange process. The influences of thermal conductivity, porosity and sphere diameter of the porous matrix, main flow temperature and heat transfer coefficient at the hot surface on temperature and saturation distributions and temperature difference within the matrix have been investigated numerically. The results indicate that a higher coolant mass flow rate can delay liquid evaporation, increase the temperature gradient in superheated vapor region and decrease the solid temperature at the hot surface, but with an increase in main flow temperature or heat transfer coefficient at hot surface, the coolant temperature increases in liquid region and especially in superheated vapor region, and the solid temperature at the hot surface increases dramatically. The results also indicate that a higher solid conductivity corresponds to a higher temperature in liquid region and in nearly the whole superheated vapor region, but a slightly lower temperature at the hot surface. A special result has been obtained that with an increase in the porosity and sphere diameter the corresponding interface of liquid region moves leftwards with the two-phase region extended, and the coolant temperature decreases in two-phase region and superheated vapor region, while in liquid region firstly it decreases and then increases, as may be determined by both heat transfer and especially pressure drop which varies dramatically with porosity and sphere diameter. The thermal non-equilibrium characteristics have been analyzed, and the results show that it is obvious at the cold surface, at the hot surface and at the beginning and the ending of two-phase region, and it is most obvious near the ending of two-phase region, and the coolant temperature is higher than the solid temperature at the beginning of two-phase region.

Effect of Transient Heat Transfer of a Condenser on a Cascade Heat Pump Performance

Transient heat transfer characteristics of a condenser of a cascade heat pump are investigated by comparing the experimental result and the prediction from a mathematical model. In the experiment part, the cascade heat pump consists of the low-temperature refrigeration cycle using R22 and the high-temperature heat pump cycle using R134a.The transient effect is presented by circulating water to receive the rejection heat from the condensing refrigerant of the cascade heat pump causing the rise of the hot water temperature. On the other hand, a mathematic model of the condensation heat consisting of the heat transfer models in the two-phase mixture region and the superheated vapor region is developed to compare the results. The effect of the hot water flow rate on the hot water temperature and the heat transfer rate is examined. The result indicates that the hot water temperature continuously increases with time. The condensation heat rapidly increases at the beginning and gradually increases with time. The result obtained from the model prediction of the condensation heat is similar to the experimental result except the initial and final period. The discrepancy between these two results is because of the transient behavior at beginning and the existent of the subcooled region.

Arresting the phenomenon of heater flooding in a wickless heat pipe in microgravity

The Constrained Vapor Bubble (CVB) is a transparent, wickless heat pipe experiment carried out in the US Labs of the International Space Station (ISS). Experiments were carried out using the 40mm CVB, 3mm×3mm in cross-section, pentane as the working fluid, with the power inputs of up to 3W. Due to the low Bond number (Bo) in microgravity and materials of construction, the CVB system was ideally suited to determine the contribution of the Marangoni forces toward the limiting heat pipe performance, and the transparent quartz shows exactly how that limitation occurs.Previous literature models and experimental temperature and pressure measurements suggested that at high enough temperature gradients, the working fluid should be subjected to enough Marangoni force to force it away from the heater and ultimately, dry out the hot end. The CVB experiment shows that high temperature gradients lead to a totally opposite behavior, i.e., ‘flooding’ of the heated end. Flooding of the heater end is attributed to a competition between Marangoni-induced flow due to high temperature gradients at the heater end and capillary return flow from the cooler. This creates a thick liquid layer in the corner of the cuvette at the heater end. At the point of flow balance, a thick layer of liquid is observed on the flat surface of the quartz cuvette. This is defined as the central drop. The region from the top of the heater end to the central drop is referred to as the interfacial flow region. The interfacial flow region develops at a power input of around 0.7W, and increases in length to the power input of 2W. At 2W, the strength of the Marangoni forces saturate. As a result, the forces in the flooded interfacial region are not able to push the liquid further into the capillary region and a further penetration of liquid down the axis of the heat pipe is arrested. As the power input is increased to nearly 3W, an increase in the vapor space is observed near the heater end at 3W. This behavior suggests that the flooding might just be an intermediate stage in reaching the dry-out limitation.The flat quartz surface at the hot end is covered by a wavy thin liquid film due to the interfacial forces. The hot end region closest to the heater is a superheated vapor region that leads to the condensation. This additional observation is discussed in Appendix.

Experimental study of pressure drop oscillations in parallel horizontal channels

Two-phase flow instabilities are an undesirable phenomenon present in many fields and scales, ranging from large heat exchangers and boilers in industrial applications to micro-scale heat exchangers for high density power electronics. In the present study pressure drop oscillations in a two parallel horizontal channels system have been experimentally investigated, focusing in the individual behavior of each channel. The balanced (same characteristic pressure drop curve) and unbalanced cases have been analyzed finding different limit cycles than the typical single channel case. No pressure drop oscillations with both channels following the typical limit cycle were found. The oscillation mode detected consisted in one channel performing the usual limit cycle, while the other was always oscillating in the superheated vapor region.

PVTx measurements of the carbon dioxide + trans-1,3,3,3-tetrafluoroprop-1-ene binary system

In this work, the PVTx properties of the carbon dioxide+1,3,3,3-tetrafluoroprop-1-ene were measured by an isochoric apparatus.The experiment covered both two phase and superheated vapor regions and have been performed a total of 8 isochores within a temperature range that spanned from (233 to 363)K and within a pressure range from (104 to 1909)kPa.Our experimental results were correlated with the Carnahan-Starling-DeSantis Equation of State (CSD EoS) and compared with REFPROP 9.1 prediction. Coefficients for CSD EoS of the system components were derived from the experimental data. Binary interaction parameter was tuned to PVTx data in the two phase region. Results in the two phase region showed a sufficient agreement with the CSD equation and with the REFPROP 9.1. Data in the superheated vapor region were also predicted extrapolating coefficients for CSD EoS at higher temperatures. Agreement with CSD EoS for the superheated vapor region experimental data was always within ±1%.The VLE behaviour of the studied binary system from the experimental data was also derived.

MS Excel Spreadsheet Add-in for Thermodynamic Properties and Process Simulation of R152a

Microsoft Excel add-in has been developed for the thermodynamic properties of refrigerant DiflouroEthane (R152a) an alternative refrigerant to R134a, which has no Ozone Depletion Potential. Thermodynamic properties’ equations for R152a were formulated from a popular equation of state. The equations were transformed into a computer program in Microsoft Excel-Visual Basic for Application in Integrated Development Environment as an Excel add-in. The add-in is able to compute the thermodynamic properties of R152a refrigerant - specific volume, internal energy, enthalpy, and entropy in the wet, subcooled and superheated-vapor regions. The calculated values are accurate compared to the standard reference properties tables for refrigerants. Computed properties’ data can easily be used in the Excel spreadsheet for process analysis, simulation and design of R152a refrigerating plants. The solution scheme and computer language adopted in this work are easy to apply and use as opposed to the available sophisticated and expensive computer software packages; and the MS Excel add-in presented in this paper would be useful to both practicing engineers and engineering students in the area. An application was illustrated by solving a typical problem in R152a refrigerating process analysis.

Numerical analysis of pressure drop oscillations in parallel channels

In this study pressure drop oscillations in two parallel channels are analyzed taking into account the thermal capacity of the pipes. A different limit cycle than the one that takes place in a single channel system is found. During the instability one channel always follows the typical pressure drop oscillations limit cycle while the other channel oscillates always in the superheated vapor region. This behavior leads to very high wall temperatures at the outlet of the heated pipe. This undesirable situation with one channel operating in the superheated vapor region takes place also for the maldistributed stable solutions.