Low Subcooling Research Articles

The effect of subcooling on hydrate generation and solid deposition with two-phase flow of pure water and natural gases

Deepwater oil and gas transportation systems play a crucial role in energy supply, but also face the challenges of hydrate generation and blockage. The study observed hydrate generation and deposition at different subcooling degrees and flow rates by using a high-pressure fully visualized recirculation pipeline. It was found that the subcooling degree has a nonlinear positive correlation with the total amount of hydrate generated. The hydrate formation at 11.7 °C subcooling was 4.5 times higher than that at 9 °C subcooling. According to the hydrate conversion rate analysis, the hydrate growth process under low subcooling conditions directly transitions from the bubble-promoting zone to the termination point of hydrate generation. Combined with the deposition state of the pipe wall and the differential pressure change curves, the risk of blockage at different subcooling levels and flow rates can be assessed.

Experimental investigation on hydrated salt phase change material for lithium-ion battery thermal management and thermal runaway mitigation

To efficiently ensure the suitable temperature for lithium-ion batteries, battery thermal management system based on phase change material (PCM) have gained considerable attention. A shape-stabilized composite PCM was prepared using the mixture of Na2SO4·10H2O and KAl(SO4)2·12H2O hydrated salts as phase change substrate, expanded graphite as thermal conductive filler and open-cell polyurethane foam as support material. The composite PCM containing 4 wt.% expanded graphite displayed a satisfied phase change temperature of 30.8 °C and 69.8 °C, high latent heat of 226.9 J/g, and good thermal conductivity of 1.39 W/(m·K). Meanwhile, it obtained a low subcooling degree, good leakage resistance, and excellent flame retardancy properties. Under PCM cooling in sandwich structure, during 0.5C charging and 1.5C discharging cycles, the maximum temperature and maximum temperature difference of battery were 52.47 °C and 2.20 °C, decreased by 20.4 % and 59.7 % respectively compared with nature air cooling. Furthermore, the composite PCM could significantly relieve the thermal runaway propagation between lithium-ion batteries. This work provides a feasible design and application strategy of hydrated salt based PCMs for battery thermal management and thermal runaway mitigation.

Study of Sintering Behavior of Methane Hydrate Particles on the Wall Surface.

The sintering of hydrate aggregates on the pipe wall is a major form of hydrate deposition. Understanding the sintering behavior of hydrates on the wall is crucial for promoting hydrate safety management and preventing pipeline blockage. However, limited research currently exists on this topic. In this study, the cohesive force strength of hydrate particles on the wall surface under different conditions was directly measured using a high-pressure micromechanical force device (HP-MMF). Subsequently, the effects of subcooling and glycine on the cohesive force were investigated. The results indicate that the cohesive force is influenced by different growth states during the process of free water on the wall surface gradually growing into hydrate. Three states with larger measured values during the growth process were selected for research. Observation showed that increased subcooling strengthened sintering by accelerating the growth rate of the hydrate film, resulting in a significant increase in cohesive force. The role of glycine in the methane hydrate system was then evaluated. Glycine was found to reduce the degree of sintering by reducing the growth rate of the hydrate film, thereby decreasing the cohesive force. The optimal concentration in the system was determined to be 0.25 wt %. Moreover, compared with low subcooling (1 °C), glycine had a better effect at high subcooling (5 °C). At 5 °C subcooling and the optimal concentration, the cohesive force in the wall droplet state decreases from 677.38 to 489.02 mN/m, the cohesive force at the low-saturation state decreases from 951.79 to 543.32 mN/m, and the cohesive force at the high-saturation state decreases from 1194.95 to 641.76 mN/m. These findings contribute to a better understanding of the cohesive force behavior of gas hydrate on the inner wall of the pipeline and provide basic data for reducing the risk of hydrate blockage.

Numerical investigation on critical heat flux for 5 × 5 fuel assemblies with a wide range of working conditions

To determine the safe boundary of fuel assembly operation and improve the heat transfer performance of fuel assembly, it is necessary to accurately calculate the thermohydraulic characteristics and the critical heat flux (CHF) in rod bundles. The Eulerian two-phase model coupled with extended wall boiling model was used to numerically investigate the 5 × 5 fuel rod bundle flows with four sets of mixing vane spacer grids. A wide range of working conditions were analyzed to validate the numerical method. The results indicate that the current numerical model can not only accurately calculate the CHF values but also predict the location of heat transfer crisis. Based on CFD (Computational Fluid Dynamics) results, the detailed two-phase flow distributions were provided to analyze the CHF performance under special structures, such as bowed rod and spacer backward shift. The quantitative results were obtained on the analyses of spacer position influence on CHF and the penalty effect of bowed rod. Furthermore, the prediction of present numerical model under low inlet subcooling conditions and cold rod CHF were discussed. The research of this paper provides support for the design and optimization of fuel assemblies.

Experimental study on flow boiling of HFE-7100 in rectangular parallel microchannel

With the rapid development of microelectronic technology, the integration and power of chip are increasing. Heat dissipation with high heat flux in limited space has become a bottleneck restricting the efficient and stable operation of the microelectronic devices. Flow boiling in microchannel heat sink is one of the most essential candidates for solving this problem. It has been shown that remarkable high heat transfer performance can be achieved through the liquid-to-vapor change process, which can dissipate a large amount of heat from a small area. In addition, dielectric fluorinated fluids, such as HFE-7100, HFE-7200, and FC-72, are especially suitable for cooling microelectronic devices, because of their excellent safety and environmental characteristics. However, dielectric fluorinated fluids have poorer thermophysical properties than water. Thus, the flow boiling heat transfer characteristics of dielectric fluorinated fluids can be different from those of water. In this work, flow boiling heat transfer and flow characteristics of HFE-7100 in a rectangular parallel microchannel are investigated. The tests are conducted at mass fluxes from 88.9 to 277.8 kg·m<sup>–2</sup>·s<sup>–1</sup>, inlet subcooling temperature from 20.5 to 35.5 ℃ and effective heat flux from 12 to 279 kW·m<sup>–2</sup> at nearly atmospheric pressure. The effects of mass flux, inlet subcooling temperature, effective heat flux and vapor quality are examined and analyzed. Additionally, flow visualization is also obtained to explain the heat transfer mechanism during the experiments. The results show that the boiling hysteresis is observed for HFE-7100 at low inlet subcooling temperature, and the increasing inlet subcooling temperature and mass flux can delay the onset of nucleate boiling. The increases of inlet subcooling temperature and mass flux can enhance the two-phase heat transfer coefficient. And the two-phase heat transfer coefficient is significantly dependent on the inlet subcooling temperature in the slug flow, while it is significantly dependent on the mass flux in the annular flow. The two-phase pressure drop increases drastically as the effective heat flux increases. And the two-phase pressure drops with different mass fluxes at constant vapor quality are obviously different between the slug flow and the annular flow. Furthermore, the experimental data are compared with four predicted values of the literature. It is found that the correlation of Lockhart has the best statistical agreement with an MAE of 19.6% and over 85% of points in the deviation bandwidth of ±30%. The results in this paper give valuable theoretical guidance for designing and optimizing heat dissipation equipment for microelectronic devices. By utilizing HFE-7100 as the coolant and microchannel heat sinks in flow boiling, it is possible to enhance the stability and reliability of the electronic devices. Additionally, the heat transfer performance associated with different heat fluxes can be improved by regulating the inlet subcooling and mass flow rate. Finally, the two-phase pressure drop correlation proposed by Lockhart can be employed to predict the pump power for heat dissipation equipment.

Double-layered phase change materials featuring high photothermal conversion for stable thermoelectric power generation

Organic solid-liquid phase change materials have attracted great attention in the field of photothermal conversion and energy storage due to their advantages such as high latent heat, low subcooling, stable chemical properties, and constant temperature during the phase change process. However, preparing organic composite phase change materials (CPCMs) with multiple desirable properties such as good shape stability, high thermal conductivity, high enthalpy, and excellent photothermal conversion efficiency, remains a challenge. Herein, using a rapid melt blending of expanded graphite (EG) into lauric acid (LA) and cold pressing technique, we constructed double-layered and shape-stabilized D-LA/EG composites. The upper layer consists of 20 wt% EG and 80 wt% LA, while the lower layer 6 wt% EG and 94 wt% LA. It was found that the prepared D-LA/EG exhibited good shape stability, high thermal conductivity, high enthalpy, and excellent photothermal conversion efficiency. More interestingly, when applied as a heat source for an all-solid-state thermoelectric generator in a photo-thermal-electric conversion system, a stable output voltage of 315 mV is generated after the light is ceased, which is attributed to the release of the heat stored in the CPCMs. This work provides a simple and economical strategy to synthesize and balance the above four properties CPCMs, which has potential applications in photo-thermal-electric conversion and solar energy utilization.

Condensation of refrigerant R450A inside a mini-channel of rectangular cross section with an internal longitudinal groove

The increase in jeopardizing the natural life and environmental resources together with high rates of energy consumption are still in focus of international agencies and scientific communities. For this issue, global awareness has been oriented to uncover alternatives to harmful species and to magnify effectiveness of thermal systems. For the present study, the refrigerant R450A was investigated as a potential substitute to R134a during condensation inside two mini-channels which are rectangular cross section channel (SRC) and a modified channel (MRC). The latter was characterized by an internal longitudinal groove. Wall subcooling, vapor superheat, and mass velocity were considered while the condensing temperature was kept constant. Numerical CFD simulation was realized for the condensation inside the channel using ANSYS-CFX 15.0 with additional user defined expressions to describe condensate layer thickness inside the channel, which in turn was utilized for the condensation heat transfer coefficient (HTC). Predicted data was compared with experimental one in terms of HTC and flow patterns. The predicted data of HTC showed an average deviation by about 6.6% compared to experimental data for R134a.The refrigerant R450A resulted in lower condensation HTC than R134a. The MRC resulted in an average enhancement in the HTC by about 55.7% and a reduction in the pressure drop by about 10.4% compared to the SRC. The enhancement of the MRC in HTC was remarkable at high degree of superheat, low mass velocity, and low wall subcooling. Accordingly, the novel MRC represents a successful potential design for condensation of refrigerants in mini-channels.

Experimental study on effects of non-ionic anti-agglomerants in preventing deposition of hydrate particles microscopically and macroscopically

Non-ionic anti-agglomerants (AAs) are important agents to prevent hydrate deposition. In this study, the microscopical experimental results show that the adhesion force between hydrate particle and droplet on wall surface mainly depends on the force of the liquid bridge under low subcoolings; while it is dependent on the resultant force of the liquid bridge and hydrate shell under high subcoolings. Below the concentration of 2 wt%, the optimal concentrations of CDEA and AEO-9 to reduce the adhesion are 0. 5 wt% and 0. 1 wt%, respectively. The adhesion force decreases gradually with the increase of their concentrations when their concentrations are lower than the optimal values, but gradually rises with the increase of their concentrations when higher than the optimal values. Moreover, the hydrate formation experiments in a stirred reactor show that the feedback torques are the smallest at the optimal concentrations of them, which is consistent with the results of the microscopical experiments.

Investigation of modeling challenges of PEM fuel cells cold start operation

In this study a one-dimensional transient isothermal model was implemented in MATLAB to conduct an in-depth evaluation of available modeling framework for cold start simulation of PEMFC. In this regard, several shortcomings were addressed and a combination of modifications introduced by other researchers and assumptions made in this study was used to improve modeling accuracy; reaction kinetics variation with temperature and ionomer water content was investigated and incorporated in the model; dynamics of nucleation and subcooling was described by Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Classical Nucleation theories; effect of temperature on ECSA loss was examined and assumptions were made regarding electro-osmotic drag in catalyst layer under low load conditions. The results showed high fidelity to experiments in the early stage of cold start and the model was able to capture the effect of all three influential parameters, i.e. temperature, initial ionomer water content, and current density on performance evolution and failing time in isothermal cold starts with acceptable accuracy. Moreover, the linear performance drop at low subcooling and the nonlinear relation between cold start duration and initial ionomer water content were properly reproduced by the model.

Numerical Investigations on Ledinegg Instability in Single and Parallel Channels under Localized Heat Source

Two-phase flow is widely used in the cooling system of electronic equipment, where the heat source is localized. However, two-phase flow may lead to flow instabilities, which have adverse effects on the system such as heat transfer deterioration and vibration. In this study, the effects of related variables on Ledinegg instability in single and parallel channels under localized heat source are investigated. Both models of single and parallel channels have been verified, and the maximum relative error is within ±5%. Based on these two models, the results indicate that the high heating power and low inlet subcooling promote Ledinegg instability, whereas Ledinegg instability is independent of the channel inclination. The average negative slope of continuous heat source, −0.09557, is smaller than that of the heat source close to inlet, −0.07408, which shows the improvement of localized heat source on Ledinegg instability. For the parallel system, when the flow excursion occurs, the total mass flow rate, which is 0.028 kg/s, is equal to two times of the mass flow rate of maximum value in branch pressure curve at which pressure is equal to 0.778 MPa, if two branches are identical. The current work provides qualitative conclusions on how Ledinegg instability will be affected with the change of relevant variables.

Exploring heat transfer efficiency in non-boiling spray cooling

Spray cooling today is one of the most effective methods for high heat flux application. Since there are numerous factors influencing the heat transfer in spray cooling, some issues related to the effect of the nozzle-to-surface distance, heat flux and liquid subcooling on the heat transfer efficiency in non-boiling mode remain debated. This study is dedicated to a comprehensive experimental investigation of the impact of various factors, including the distance from the nozzle to the heater (2–35 mm), heat flux density (up to 6.9 MW/m2), liquid flow rate (8.8–25.2 mL/s), and initial liquid temperature (20 and 80 °C) on heat transfer in non-boiling spray cooling using different nozzles with varying spray angles (30–90°). Heat transfer results were obtained based on field temperature measurements using high-speed infrared thermography. It is shown that the nozzle-to-surface distance has a significant effect on the spatial distribution of the temperature field and the heat transfer rate in spray cooling. It is shown that for each nozzle and heating surface there is an optimum distance at which the maximum heat transfer rate at non-boiling spray cooling is observed and relationship for their determination are proposed. The heat transfer coefficient for deeply subcooled liquid (80 K) is practically independent of the heat flux. At low subcooling levels (20 K), the heat transfer coefficient increases with the increase in heat flux associated with the influence of evaporation from the liquid film surface on the total heat transfer. The results obtained are analyzed, compared and generalized with data from the literature, and a correlation is proposed for determining the heat transfer in non-boiling spray cooling.

Study on two-phase flow instability of parallel helical tubes in steam generator of small modular reactors

The flow instability phenomenon within parallel helical tubes is experimentally studied. The oscillation characteristics of wall temperature are comprehensively discussed. In the experiment, the pressure of the test section is in the range of 2.7–6.0 MPa, the mass flux is in the range of 100–300 kg/m2s, the degree of inlet subcooling is in the range of 10–75 °C, and the inlet throttling is in the range of 82–328. This study reveals different effects of inlet subcooling. Specifically, increasing the inlet subcooling under high mass flux enhances the stability of parallel helical tubes. However, increasing the inlet subcooling under low mass flux and low subcooling weakens the stability. The oscillation period to mixture transit time ratio is small at high inlet subcooling and increases with decreasing subcooling. Furthermore, this study shows that the onset of wall temperature oscillation is closely related to the dryout phenomenon, and that the oscillation amplitude increases with increasing local quality. Finally, this study demonstrates that the flow instability can be accurately predicted using time-domain method. The simulation results show satisfactory accuracy, with the prediction errors of ± 15% for the instability threshold and oscillation period, and ± 20% for the wall temperature oscillation amplitude.

A coupled level-set and volume-of-fluid (CLSVOF) method for prediction of microgravity flow boiling with low inlet subcooling on the international space station

The present study is part of a series of NASA-supported investigations of flow boiling of n-perfluorohexane (n-PFH, C6F14) under microgravity on the International Space Station (ISS). The fluid physics and heat transfer characteristics of flow boiling with low inlet subcooling are explored using the Flow Boiling and Condensation Experiment (FBCE) (which is actual name of the test facility), the largest endeavor in microgravity two-phase research conducted to date. Experimental data are collected from the FBCE's Flow Boiling Module (FBM), which features a rectangular channel with 5.0-mm x 2.5-mm cross-section and 114.6-mm heated length. Results for three flow rates are used to explore the effects of mass velocity on flow structure and heat transfer characteristics in the absence of the body force. A Computational Fluid Dynamics (CFD) model is constructed to predict the measured FBM results. The CFD model employs the Coupled Level-Set and Volume-of-Fluid (CLSVOF) method, which is modified with improved interface capture and additional force terms in the momentum equation to determine the bubble dynamics more accurately. Validity of the CFD model is assessed in two ways, by comparing predictions against video-captured interfacial behavior and heat transfer data. The CFD model shows good ability to capture detailed evolution of the interfacial structure both across and along the flow channel, including bubble nucleation, growth, departure, and coalescence, as well as axial development of dominant flow patterns. Details of the flow structure are examined via axial development of both flow velocity and void fraction profiles. Similarly, heat transfer results are presented in terms of streamwise variations of both wall temperature and fluid temperature, which are also predicted accurately with the CFD model. Overall, this study proves the CFD model is an effective method for both design and performance assessment of flow boiling subsystems in space vehicles.

Dropwise condensation heat transfer on vertical superhydrophobic surfaces with fractal microgrooves in steam

Despite dropwise condensation witnesses the superior heat transfer performance with in-time removal of condensate, pinning and flooding phenomena significantly limit its application at high subcooling. In this study, we design and fabricate a self-similar fractal groove superhydrophobic surface inspired by the self-similar fractal spines of cactus thorns. A visualization and condensation heat transfer measurement experimental platform is established to investigate droplet dynamics and heat transfer performance on the vertical self-similar fractal groove superhydrophobic surface. Dropwise condensation is analyzed and compared with that on plane and groove superhydrophobic surfaces. The fractal grooves promote spontaneous coalescence-induced jumping of mismatched condensate droplets at low subcooling, resulting in a higher probability and frequency of droplet jumping. Furthermore, second-level grooves constrain the shape of large droplets more effectively and facilitate condensate removal after the failure of first-level grooves at high subcooling. In agreement with droplet dynamics, the heat transfer measurements demonstrate that the fractal groove superhydrophobic surface achieves the highest condensation heat flux and heat transfer coefficient among the three investigated superhydrophobic surfaces as subcooling increases.

Novel low-cost steel slag porous ceramic-based composite phase change material: An innovative strategy for comprehensive utilization of steel slag resources

This work creatively proposes a novel and low-cost steel slag porous ceramic (SSPC)-based NaNO3 salt composite phase change material (PCM) using industrial solid waste steel slag as the main raw material. In this study, the foaming-gel casting process prepares SSPC with high porosity (77–83%) and mechanical strength (0.36–1.13 MPa), which shows good adsorption of NaNO3 salts. The spontaneous infiltration method prepares and characterizes the NaNO3/SSPC composite PCM by performance properties. The results show that the NaNO3 salt is uniformly adsorbed in the pores of SSPC with good chemical compatibility, and the maximum filling rate reaches 86.26%. The porous media can effectively prevent the leakage of molten NaNO3 salt. The prepared composite PCMs have lower melting and freezing temperatures, lower subcooling by 5.47 °C, and higher thermal conductivity by 121.6% compared to pure NaNO3 salt. The latent heat retention of the composite after 50 thermal cycles is more than 96%, with good thermal reliability. Finally, a prospective process of using steel slag to prepare composite PCMs for collecting and storing waste heat from the steel industry is proposed. Reusing steel slag and storing waste heat from the steel industry are realized, both beneficial for sustainable development.

Investigation on microscopic forces between methane hydrate particles in gas phase dominated system

Hydrate aggregation and deposition on the pipe wall are the main causes of hydrate plugging in pipelines. Microscopic forces between hydrate particles are an important reference for determining the behavior of aggregation or separation from the pipe wall of hydrate particles. Based on a high-pressure visual micro-force measurement system, this study measured the hydrate particle-hydrate particle cohesive force and hydrate particle–wall liquid droplet adhesion force under pure water, as well as hydrate particle–wall liquid droplet adhesion force under anti-agglomerants (AAs). It was found that the magnitude of hydrate particle-hydrate particle cohesive force was ×10 mN/m, and the magnitude of hydrate particle–wall liquid droplet adhesion force was ×103 mN/m. The effects of 4 AAs in the methane gas-dominated system were evaluated from a micro-perspective, revealing that after adding DDBAB, D1021, AEO-9, and CDEA, the maximum reduction of adhesion force was 67%, 64%, 51%, and 53% compared with that under pure water, respectively. The mechanism of AAs reducing adhesion force was further explored, revealing that the adhesion force arose from a liquid bridge and was affected by the gas–water interfacial tension of solutions at low subcoolings. While at high subcoolings, the force was determined by the combined force of the liquid bridge and the hydrate shell. The addition of AAs reduced the gas–water interfacial tension of solutions, and the steric hindrance effect of AA molecules also weakened the tensile strength of the hydrate shell, leading to a decreased adhesion force. These experimental data can help deepen our understanding of the microscopic forces between methane hydrate particles, and provide experimental data and scheme design basis for wellbore flow assurance of deepwater gas wells.

Condensation characteristics of air–water vapor mixture on the surface of vertical flat plate

The condensation heat transfer of air–water mixed vapor is an important heat transfer phenomenon in production life. An experimental system for condensation heat transfer of the air–water vapor mixed vapor is built in this paper. The condensation characteristics of the mixed vapor and the double boundary layer structure characteristics were investigated. The study mainly discussed the influence of flow velocity on the condensation performance of mixed vapor and summarized the experimental correlation equations considering flow velocity. Results indicate that the condensation performance decreases significantly as the air concentration increases. The mixed vapor flow velocity was increased from 1 to 3 m/s, and the condensation heat transfer coefficient was increased by 34.9–121.0%. The enhanced effect of flow velocity was also observed at high air concentration and low subcooling. The ratio of convective and condensation heat transfer coefficients increases with the rise in air concentration, reaching a maximum of 0.12. The proportion of convective heat transfer can be reduced by increasing the mixed vapor flow velocity. The gas film thickness is substantially higher than the liquid film thickness in mixed vapor condensation, and the gas/liquid film thermal resistance ratio can be more than 30 times. And increasing the flow velocity can significantly reduce the gas film/liquid film thermal resistance ratio. A new experimental correlation equation is fitted based on the experimental results, and this equation can be used as a reference for the design of related heat exchangers.

Synergistic Kinetic Hydrate Inhibition of Pectin, PVP, and PVCap with Monoethylene Glycol

Before injecting any kinetic hydrate inhibitor (KHI) into a flow line, the standard procedure is to dissolve the chemical, which is mostly a polymer, in a solvent such as water, methanol, or glycol. It is ideal if the solvent synergizes with the KHI in hydrate inhibition. In this paper, the synergy of three KHIs, poly(vinylpyrrolidone) (PVP), poly(vinyl caprolactam) (PVCap), and a natural biodegradable polymer Pectin at 0.25 wt %, is investigated with one of the common solvents monoethylene glycol at concentrations (0.5–20 wt %). The hydrate-inhibiting performance of the KHIs and their solvent blends is evaluated using an autoclave to measure the induction time (IT) and the molar hydrate growth rate, which is approximately taken equal to the molar gas consumption rate (HGR/GCR) in constant cooling rate tests and isothermal tests. The individual hydrate inhibition testing results showed that MEG alone at high dosages (up to 20 wt %) is a very poor KHI, while Pectin is a moderate KHI with performance lower than the commercial KHIs (PVP and PVCap) at similar concentrations. The hydrate growth rate of Pectin is however, lower compared to that of both PVP and PVCap. The blends of the KHIs with MEG, showed significant hydrate inhibition synergy in all of the KHI–MEG blends with increasing MEG concentration from 0.5 to 20 wt %. The synergy was maximum at 5 wt % of MEG for all the KHIs with IT enhancement of around 50% for PVP and PVCap and 200% for Pectin. Almost 30–60% reduction in HGR was observed in the blends compared to that of their individual KHI polymer HGRs. Isothermal experiments at 1.5, 6, and 8 °C were done using KHI–MEG blends to simulate possible field/subsea conditions. Among the three KHIs at 0.25 wt % blended with 5 wt % of MEG at 6 °C, PVP–MEG and Pectin–MEG blends gave an almost equal IT of 3.5 h, while the PVCap–MEG blend gave a slightly higher IT of 4.55 h. The HGR of the Pectin–MEG blend was however, the lowest (0.055 m/h) when compared to the PVP–MEG blend HGR (0.08 m/h) and the PVCap–MEG blend HGR (0.075 m/h). Similar IT but lower HGR values were found at 1.5 °C when 20 wt % of MEG with 0.25 wt % of Pectin was used. No hydrates were formed up to 6 days with 0.25 wt % of Pectin and 5 wt % of MEG at 8 °C due to a low subcooling of 1.3 °C. In summary, adding MEG to commercial KHIs such as PVP, PVCap and natural KHI Pectin increases their IT tremendously due to synergy and at the same time lowers their HGRs. As the blends of Pectin–MEG are highly biodegradable, they can be used at locations that have relatively low subcooling requirements and where biodegradability and lower hydrate growth rate are essential as they have similar ITs as that of PVP–MEG blends while lower HGRs than both PVP–MEG and PVCap–MEG blends.

Effect of wall subcooling on condensation in a steam-air mixture on a vertical tube

In this study, the effects of wall subcooling on the condensation heat transfer in steam-air mixture on a vertical tube was investigated. The effect of wall subcooling on condensation heat transfer was analyzed experimentally and using heat and mass transfer model. The experimental analysis indicated that wall subcooling effect varied depending on the degree of subcooling, i.e., the decrement of condensation heat transfer became less in low subcooling. In addition, the noncondensable gas fraction affected the wall subcooling effect. Such phenomenon was also found in the analysis using heat and mass transfer model. Through analysis using heat and mass transfer model, it was found that the accumulation of noncondensable gas and steam diffusion have competing effects. The former deteriorates and the latter enhances the condensation heat transfer with increasing wall subcooling. In low subcooling condition, both effects have higher rate of change, resulting in the different subcooling effect with that of high subcooling condition. We then developed an empirical correlation of condensation heat transfer over a wide range of wall subcooling; this predicted the experimental results reasonably well, particularly at low subcooling.

Experimental investigation of the effects of metal oxides and nucleating agents on nano-emulsions heat transfer performance in mini-channels

Recently, thermal conductivity, even and stable suspension of particles, and agglomeration of particles have been studied comprehensively. These properties are especially important parameters for nanofluids in small-size heat transfer devices like mini-channels, which are prone to block due to the small size of the channels. In the present study, a method was proposed to increase heat transfer using composite nano-phase change emulsions (NPCEs) slurries, utilizing the latent heat of PCM during melting. In this experimental study, a series of composite nano-phase change emulsions (NPCEs) with high thermal conductivity, high thermal storage, and low supercooling were developed and prepared from hexadecane, octadecane, hexadecanol, octadecanol, and nano-Al2O3. Heat transfer experiments were conducted in a mini-channels using the prepared NPCEs as working fluids. The effects of the nanoparticle concentration, NPCE type, and flow rate on multiple parameters, including the heat transfer coefficient, flow resistance, and heat transfer performance of composite NPCEs were investigated. As a result, the heat transfer experiments demonstrated that the proposed NPCEs as heat transfer fluids could enhance the heat transfer performance in mini-channels in comparison with deionized water. Compared with deionized water, the convective heat transfer coefficient was increased by a maximum of 1.28 %, 4.07 %, and 4.19 % with an octadecane concentration of 5 wt%, 10 wt%, and 20 wt%, respectively. The nano-emulsion with a combination of 5 wt% hexadecane, 5 wt% octadecane, and 1 wt% nano-Al2O3 achieved a maximum increase of 7.01 % in the heat transfer enhancement factor. In addition, the nano-emulsion with a combination of 5 wt% hexadecane, 5 wt% octadecane, and 1 wt% nano-Al2O3, in addition to 1.25 wt% hybrid nucleating agents allowed a maximum increase of 26.49 % in the heat transfer enhancement factor. The obtained results and performed analyses demonstrate that metal particle-composite alcohol hybrid nanofluid has high stability, high thermal conductivity, low subcooling, high durability, and a convenient preparation method, and is an appropriate choice as a working fluid, especially in small-scale heat transfer devices.