Evaporative Heat Research Articles

The Effect of Air Parameters on the Evaporation Loss in a Natural Draft Counter-Flow Wet Cooling Tower

The measures to reduce the impact of evaporation loss in a natural draft counter-flow wet cooling tower (NDWCT) have important implications for water conservation and emissions reduction. A mathematical model of evaporation loss in the NDWCT was established by using a modified Merkel method. The NDWCTs in the 300 MW and 600 MW power plant were taken as the research objects. Comparing experimental values with calculated values, the relative error was less than 3%. Then, the effect of air parameters on evaporation loss of NDWCT was analyzed. The results showed that, with the increase of dry bulb temperature, the evaporation heat dissipation and the evaporation loss decreased, while the rate of evaporation loss caused by unit temperature difference increased. The ambient temperature increased by 1 °C and the evaporation loss was reduced by nearly 26.65 t/h. When the relative air humidity increased, the evaporation heat dissipation and evaporation loss decreased, and the rate of evaporation loss caused by unit temperature difference decreased. When relative air humidity increased by 1%, the outlet water temperature rose by about 0.08 °C, and the evaporation loss decreased by about 5.63 t/h.

Salt Mitigation Strategies of Solar‐Driven Interfacial Desalination

AbstractSolar‐driven interfacial desalination (SDID), which is based on localized heating and interfacial evaporation, provides an opportunity for developing environmentally friendly and cost‐effective seawater thermal desalination. However, localized heating and rapidly generated interfacial steam may cause salt to accumulate on the evaporator's surface and block the channel of steam evaporation. Salt accumulation inevitably reduces the light absorption and service period of the solar absorber, resulting in a significant decrease in evaporation efficiency over time. Salt accumulation makes it difficult to produce SDID devices with high energy efficiency and long‐term stability for large‐scale use in remote poverty‐stricken areas. Therefore, the exploration of novel and effective strategies for addressing salt accumulation through both material design and structural engineering has attracted more attention in recent years. This review presents an overview of the state‐of‐the‐art advancements in salt‐resistant photothermal evaporation and discusses the critical issues for achieving salt mitigation SDID, focusing on the classification of salt mitigation strategies based on photothermal evaporation configurations, the basic mechanism of salt mitigation, and the architectural design of photothermal materials. Finally, the important challenges and prospects of SDID are discussed to providing a meaningful roadmap to efficient salt mitigation SDID.

Analytical and experimental study on boiling vaporization and multi-mode breakup of binary fuel droplet

In this paper, the boiling vaporization and breakup of single droplets of n-butanol, n-hexadecane and their binary mixtures with 10-50 w.t.% n-butanol have been studied. The experiments of droplet evaporation with an elevated ambient temperature range of 537-609 K were conducted through the pendant drop method and high-speed camera technology. The mathematical method was also proposed to predict the heterogeneous superheat limit and nucleation rate within the droplet. The experimental results indicate that mono-component droplets only go through the transient heating and stable evaporation phases. While for bi-component droplets, the characteristics show strong two-phase flow instability. With increasing n-butanol concentration or ambient temperature, the heterogeneous nucleation will occur inside the blended droplet, which experiences fluctuation evaporation between the transient heating and stable evaporation phases. The droplet vaporization characteristics gradually change from normal evaporation to partial rupture or passive breakup and then to micro explosion which reduces the droplet lifetime considerably. The novelty of this work is that various ambient temperature conditions and compositions are adopted to study the droplet evaporation to identify the conditions that favor flash boiling, and a new analytical model of heterogeneous nucleation is proposed, which can be used to estimate the temperature at the instant of bubble nucleation.

Experimental study of change of average velocity in a drop on crystalization

The behavior of convection dynamics during high-temperature heating inside a drop of an aqueous solution of LiBr salt at the moment of crystalline-hydrated crust movement was studied experimentally. To measure the instantaneous velocity fields in the horizontal section of a sedentary drop, the optical PIV method was used. The measurement of convection velocity in a drop through a thick crust of crystalline hydrates did not affect the accuracy of measurements. The maximum average velocity UAv is equal to 0.39-0.42 mm/s was realized at the initial time. Over 30-35 s the temperature gradient decreases due to heating of the solution by 3.5 times which led to a decrease in UAv by 3-4 times. After 32 s heat transfer and evaporation enter quasi-stationary mode.

Simulation investigation on pre-cooled heat pipe (PCHP) for free cooling of high thermal density space

High thermal density space consumes massive energy, and cooling device accounts for large percentage of total consumption. To cut down energy consumption of cooling device, PCHP is proposed on the basis of heat pipe and evaporation technologies. Heat transfer and energy consumption models of PCHP are developed. The effects of geometric and operational parameters on the PCHP performance, including pad thickness, ambient temperature and ambient relative humidity (RH), air flow rate, are investigated. The results show that cooling capacity increases with the increase of pad thickness and airflow rate, and it decreases with the increase of ambient temperature and RH. With the increase of thickness, energy efficiency ratio (EER) increases at the start and then decreases under RH20%-40%, while it decreases continuously under RH70%-90%. EER increases with the decrease of ambient temperature, ambient RH and air flow rate. PCHP markedly enhances annual free cooling time compared with conventional heat pipe system (CHP), and the maximal enhanced free cooling time and increasing percentage occur at Urumchi and Guangzhou Cities, respectively.

Turbulent liquid film evaporation in a partially heated wall along a vertical tube

AbstractIn this numerical study, the evaporative heat and mass transfer of a turbulent falling liquid film in a finite vertical tube are investigated. The liquid film flows in the tube's inner wall, whose outer wall is partially subject to thermal flux. Here, different configurations corresponding to thermal flux imposed on different external surface wall percentages are examined. External face zones where the heat flux is not applied are maintained insulated. The nonlinear set of parabolic mass, momentum, energy, and mass fraction conservation equations combined with boundary and interfacial conditions are treated numerically using implicit finite difference procedure. For falling liquid film analysis, an adapted Van Driest turbulence model is used. For the present work, it is supposed that gas flows in a laminar regime. We examine in this paper the impact of the percentage of heated surface area on flows as well as on heat and mass transfer. Obtained results for a partially heated wall are compared with those produced for an entirely heated wall.

Evaporation and Combustion Characteristics of Kerosene Droplets in Localized Stratified Vortex-tube Combustor: A Numerical Investigation

ABSTRACT The combustion of AC10H20 droplets was investigated to explore the combustion performance of liquid fuels in localized stratified vortex-tube combustor (LSVC). The evaporation ratio, flame structure, stability limit, heat loss, and combustion efficiency in the LSVC were investigated under various equivalence ratios and fuel mass fluxes numerically. Results corroborate that the LSVC exhibits a large heat release with uniform flame front, large stability limit, low heat loss, high evaporation rate, and good combustion efficiency under lean operating conditions, indicating good potential to deal with liquid fuels directly. Then, the evaporation and stabilization mechanisms are analyzed. As for the former, the evaporation ratio in LSVC increases sharply along the axial direction toward the outlet, indicating a high evaporation rate, which is optimized through the heat produced by itself efficiently. Viz., the vortex currents can entrain the AC10H20 droplets to interior high-temperature region and then promote the evaporation of liquid fuels. As for the latter, the localized stratified distribution of species in the LSVC results in an edge flame structure, which differs from that in the traditional vortex-tube combustors. The local equivalence ratio increases along the radial direction toward the center. The increased local equivalence ratio of the interior is crucial for stabilization and the decreased local equivalence ratio of exterior enables the heat loss to be reduced. In the end, the edge flame structure and the low heat loss yields a large heat release in the LSVC, which can increase the flame speed, thereby ensuring the stabilization and the high burn-off rate.

Integrated polygeneration system for coastal areas

Polygeneration is a sustainable energy concept, which could deliver multiple energy vectors (power, heat and cooling) and other useful products (desalinated water, hydrogen, etc.) by possible integration of suitable technologies. In this study, a polygeneration system for coastal area applications is driven by a hybrid solar/bio-oil boiler to produce the energy intensive products of cooling, power, desalinated water and hot air for drying. Solar thermal is the primary energy source to drive the polygeneration system, whereas the bio-oil boiler is the auxiliary source to drive it at non-sunshine hours. The performance characteristics of the hybrid system is investigated to identify the effects of various key parameters such as temperatures of heat source, sink and evaporator along with expander Built in Volume ratio. The obtained results at a typical operating condition show that the parabolic trough collector aperture area is 130 m2, the system produces net power output of 47 kW, desalinated water of 1.205 m3/h, 92 kW of hot air for drying and 141 kW of cooling. The overall energy and exergy efficiencies of the proposed system are found to be 22% and 87%, respectively.

Estimating lake temperature profile and evaporation losses by leveraging MODIS LST data

Global lake evaporation is a critical component of the terrestrial water cycle. Accurate quantification of lake evaporation dynamics is of high importance for understanding lake energy budgets, land-atmosphere interactions, as well as regional water availability. However, the accurate quantification of lake evaporation has been hindered by the complexity involved with addressing the heat storage of water bodies. In this study, a new model—the Lake Temperature and Evaporation Model (LTEM)—was developed to simulate lake water temperature profiles, which were then used to calculate heat storage changes and evaporation rates. Inputs for the LTEM include the meteorological and bathymetric data, as well as the Moderate Resolution Imaging Spectroradiometer (MODIS) water surface temperature (WST)—which is the land surface temperature (LST) over water. The MODIS WST was leveraged to constrain the hydrodynamic simulations. Model results over 11 lakes around the world show robust performance of LTEM. The long term average temperature biases range from -0.5 °C to 0.5 °C, and the evaporation rate biases range from -0.19 mm/day to 0.28 mm/day. In particular, it is found that LTEM significantly improves the simulation of the seasonality of lake evaporation rates. The validation results suggest that the averaged coefficient of determination (R2) for the evaporation rate is 0.84, which is 0.28 higher than that obtained when the conventional Penman equation (without heat storage) is used. The volumetric evaporation time series was then calculated as a product of the monthly evaporation rate and lake surface area (derived from MODIS near-infrared image classifications). This study provides an end-to-end framework for quantifying volumetric evaporation for the world’s lakes and reservoirs. It also provides the capability to investigate the thermal dynamics of lake systems, and thus can benefit the various water resources applications across scales.

Parametric investigation of radiation heat transfer and evaporation characteristics of a liquid droplet radiator

The liquid droplet radiator (LDR) as a frameless heat removal system is considered as a promising solution for the waste heat dissipation of megawatt aerospace applications. The radiation heat transfer and evaporation characteristics of the LDR working fluid are tightly coupled with the operational performance of the liquid droplet generator. In this paper, the effects of operational parameters of droplet generator on the radiative heat transfer and evaporation loss rate are clarified, including the pressure disturbance frequency, pressure difference between the inside and outside of the droplet generator. It is observed that higher coolant mass flow rate does not assure higher heat transfer power for the LDR. For cases with higher pressure difference, where the coolant flow rate is higher consequently, the evaporation loss rate increases continually with pressure difference while the heat transfer power does not increase any more. The disturbance frequency has inconspicuous impact on the evaporation characteristics. Besides, cases with higher pressure differences have wider suitable frequency ranges according to the droplet formation restriction. Generally, a relatively higher pressure difference coupled with a higher disturbance frequency (such as 0.3 MPa and 12 kHz) is suggested to achieve a higher heat transfer power and low evaporation loss rate. This paper may provide favorable reference for determining the operational parameters of the droplet generator in consideration of radiation heat transfer and evaporation characteristics.

Using solar collector unit in a methanol-water vapor absorption cooling system under iraqi environmental conditions

Testing, designing and fabricating a friendly vapor absorption cooling system was the main purpose of the present study using water and methanol as a working fluid. Various Iraqi environmental conditions (Hilla City) are the operating conditions under which the test rig experiment was performed. The temperature of the heat source, condenser, absorber and evaporator, is the involved property in the test calculations. The system is tested during June and July 2019. The results show that the COP of the absorption cooling system ranged from 0.22 to 0.38 while the range of temperature drop in evaporator is (9.8 °C–15.8 °C). Condensation and absorption temperatures are under 45 °C while the maximum temperature of the driving water was 85 °C. The results also show that the generator temperature had a great effect on the system performance.

Using Thermodynamic Availability to Predict the Transitional Film Reynolds Number between the Jet and Sheet Modes in Falling Liquid between Horizontal Tubes

The purpose of this paper is to develop and validate a theoretical framework -based on thermodynamic availability- that directly predicts the transitional film Reynolds numbers between the jet and sheet modes of falling films on horizontal tubes. The prediction of the prevailing flow mode is important for the design and operation of falling film heat and mass exchangers. The proposed model accounts for fluid properties as well as tube geometry (diameter and spacing). The transitional film Reynolds numbers calculated using the proposed model were validated against 52 experimental measurements from the literature. More than 73 % of the results from the proposed model are within ± 25 % from their respective experimental values. Moreover, the proposed model has a mean absolute percentage error of 20.4 % which is lower than the 30.4% obtained by the widely-used empirical model (Re=a(Ga)b), for the same validation data. The proposed model offers a convenient tool for predicting the mode transitions, provides insight into the role of availability in the mode transitions, and has the potential of integration with other models (e.g. mode-based heat transfer models, evaporator models, and chiller or desalination system models).

Molecular dynamics study on the effect of hydrophilicity on thin film evaporation over biphilic surface

The objective of this study is to understand the effect of surface wettability and how nano-scale heat transfer is affected by different surface wetting conditions over biphilic surface by molecular dynamics simulations. The system of 3 nm liquid Argon film over Platinum surface is equilibrated at 90 K and then wall temperature is raised to higher value to study the effect of temperature. According to this study at low wall temperature and low surface wetting conditions, liquid temperature increases proportionally with the increment of hydrophilic portion but at higher wall temperature and better surface wetting conditions, maximum value of liquid Argon temperature increases with increasing hydrophobic portion. Evaporation number, evaporative mass flux and heat flux increase significantly with slight increment of hydrophilic portion. Inception time decreases with increasing hydrophilic portion and surface wettability. Also, for better surface wetting conditions, average evaporative mass flux is more sensitive to wall temperature and increases significantly at higher wall temperature.

RETRACTED ARTICLE: Influence of evaporation temperature on the rheological properties of modified emulsified asphaltic residues

For two commonly used modified emulsified asphalts, styrene butadiene rubber (SBR) and styrene butadiene styrene (SBS), direct heating and low-temperature evaporation methods were respectively applied to obtain the asphaltic residues. Nanotechnology provides a great solution to road construction issues. However, the use of nanoparticles increases the durability of asphalts and enhances the performance of bitumen. Dynamic shear rheometer (DSR) was employed for temperature scanning test and multiple stress creep recovery (MSCR) test of the asphaltic residues. In this paper, the changes in the rheological properties of the asphaltic residues were compared and analyzed. The influence of evaporation temperature on the rheological properties of modified emulsified asphaltic residues obtained by evaporation was discussed. Evaporation tests showed that the higher the evaporation temperature, the larger the mass reduction of residues at the early stage and the shorter the time needed to reach stabilization. The solid contents of modified emulsified asphaltic residues obtained under different evaporation temperatures also differed. The solid content of asphaltic residues obtained under the evaporation temperature of 60 °C was closest to that by the low-temperature evaporation method. This indicated influence of the evaporation temperature on the solid content of asphaltic residues. Based on the results of temperature scanning test and MSCR test, there was a significant difference in the rheological properties of the asphaltic residues obtained by direct heating and low-temperature evaporation methods. The anti-deformation capacity under high temperature in asphaltic residues obtained at different evaporation temperatures varied more greatly. For SBS-modified emulsified asphalt, the rutting resistance under high temperature in residues obtained by the low-temperature evaporation method was better; however, the opposite was true for the SBR-modified emulsified asphalt. This indicated that the high-temperature evaporation method may overestimate the high-temperature properties of SBR-modified emulsified asphaltic residues and underestimate the high-temperature properties of SBS-modified emulsified asphaltic residues. For the emulsified asphalt samples, the appropriate method to obtain asphaltic residues that are closest to the actual road conditions should be optimized depending on the specific purposes.

Bus LNG container heat insulated performance estimate

Many buses use cryogenic container to store the LNG as fuel. The heat insulated performance is an important parameter that reflected with daily evaporation rate or heat leakage of cryogenic container. In bus running process, the LNG container is affected by many factors and difficult to assess and analyze the heat insulated performance. In order to solve this problem, firstly, according to the LNG container heat leakage transfer process in bus running, the energy equations of the LNG were established. Secondly, the factors were analyzed that affected the heat insulated of the LNG cryogenic container. Then, the heat leakage was calculated depending on the test data of the LNG container in the bus running to estimate the heat insulated performance. At the same time, the LNG container heat insulated performance was also measured at the local inspection agency. We found that there were great differences between the two results, and the reasons for the great differences were further analyzed.

Feedbacks Driving Interdecadal Variability in Southern Ocean Convection in Climate Models: A Coupled Oscillator Mechanism

AbstractNumerous climate models display large-amplitude, long-period variability associated with quasiperiodic convection in the Southern Ocean, but the mechanisms responsible for producing such oscillatory convection are poorly understood. In this paper we identify three feedbacks that help generate such oscillations within an Earth system model with a particularly regular oscillation. The first feedback involves increased (decreased) upward mixing of warm interior water to the surface, resulting in more (less) evaporation and loss of heat to the atmosphere which produces more (less) mixing. This positive feedback helps explain why temperature anomalies are not damped out by surface forcing. A second key mechanism involves convective (nonconvective) events in the Weddell Sea causing a relaxation (intensification) of westerly winds, which at some later time results in a pattern of currents that reduces (increases) the advection of freshwater out of the Weddell Sea. This allows for the surface to become lighter (denser) which in turn can dampen (trigger) convection—so that the overall feedback is a negative one with a delay—helping to produce a multidecadal oscillation time scale. The decrease (increase) in winds associated with convective (nonconvective) states also results in a decrease (increase) in the upward mixing of salt in the Eastern Weddell Sea, creating a negative (positive) salinity anomaly that propagates into the Western Weddell Sea and dampens (triggers) convection—again producing a negative feedback with a delay. A principal oscillatory pattern analysis yields a reasonable prediction for the period of oscillation. Strengths of the feedbacks are sensitive to parameterization of mesoscale eddies.

Energetic and exergetic studies of modified CO2 transcritical refrigeration cycles

Abstract Thermodynamic analysis including energetic and exergetic analysis is carried out employing Engineering Equation Solver for the five modified cycles: dual expansion cycle, internal heat exchanger cycle, work recovery cycle, work recovery with internal heat exchanger cycle and vortex tube expansion cycle. Contours are developed to study the effect of gas cooler temperatures and evaporator temperatures on the system performance and optimum gas cooler pressure. The modified cycle with work recovery turbine offers relatively higher COP and higher exergetic efficiency with lower compressor discharge pressure. The exergy loss in compressor, gas cooler, throttle valve and vortex tube (VT) are considerably higher than that in internal heat exchanger (IHX), evaporator and turbine. It is observed that COP of modified cycle with VT is slightly less than that with IHX, whereas the cycle with work recovery turbine brings the highest COP with the improvement of 25% at the gas cooler exit temperature of 305 K and evaporator temperature of 248 K.

How to evaluate the performance of sub-critical Organic Rankine Cycle from key properties of working fluids by group contribution methods?

An artificial neural network (ANN) model is developed to predict the ORC performance from key properties of working fluids, including critical temperature, critical pressure, acentric factor and ideal gas heat capacity, based on the 5400 calculated data from REFPROP for 54 working fluids. When these key properties are unknown for working fluids, group contribution methods (GCMs) are employed to combine with the established ANN. For the considered three GCM-ANN models, 21 potential working fluids of ORC are used to evaluate the accuracy in the prediction of key properties and cycle parameters. From the obtained results, it can be concluded that the developed ANN has average absolute deviations (AADs) 5.9866%, 0.1024%, 0.9684%, 0.1131% and 1.6283% for pump work, evaporation heat, turbine work, condensation heat and cycle efficiency, respectively. For key properties, accuracy of critical temperature has the most significant effect on the ORC predictions. As for the three GCMs, SU-GCM has the least deviations for the prediction of properties. The corresponding AADs of GCM-ANN model are 15.89%, 10.73%, 12.88%, 10.40% and 2.51% for pump work, evaporation heat, turbine work, condensation heat and cycle efficiency, respectively. When key properties are obtained from experiments or GCMs, the developed ANN can be applied to predict ORC performances for any working fluid easily and quickly.

On heat and mass transfer using evaporating self-rewetting mixtures in microchannels

Evaporation heat and mass transfer in open microchannels was investigated using pure liquids (ethanol, deionized water and butanol), an ordinary (non-self-rewetting) mixture (5% v/v ethanol/water mixture) and a self-rewetting fluid (5% v/v butanol/water mixture). The applied heating power ranged from 0 mW to 282.76 mW (giving heat fluxes from 0 W/m2 to 4501 W/m2) with wall temperatures ranging from 24 °C to 95 °C. When the microchannel had no external heating, diffusion was found to be the main mechanism of mass transfer. With external heating, the diffusion model underestimated the experiment results, and buoyancy-driven convection may account for this. Evaporation rates remain high for the self-rewetting fluid even when the meniscus recedes significantly (by 6 times the inner diameter) in the microchannel under heating conditions. Additionally, the self-rewetting fluid showed the highest conduction rate in the axial direction in high heating power conditions. Meniscus-wall contact angles and flow visualizations were obtained for various heating rates. The “contact angle shift” phenomenon of the self-rewetting fluid was observed. Marangoni-induced convection in the microchannel accounts for the mechanism of better heat and mass transfer as well as the “contact angle shift” phenomenon for the self-rewetting fluid.

Mathematical model and calculation results of the kerosene aerosol combustion rate

This paper presents a mathematical combustion model of the kerosene aerosol and the calculation results of the combustion front normal velocity. The model is based on the dynamic equations of the two-phase reacting medium and consists of the energy equations for the gas and kerosene droplets, taking into account heat exchange, evaporation and heat release from the kerosene-oxygen reaction in the air; mass conservation equations for the gas, droplets, oxygen and kerosene, controlling the mass exchange between the phases and the burning of the reagents; the motion equation of the droplets; the particle number equations for the of droplets per unit volume; the ideal gas equation. The model assumes the isobaric conditions of the aerosol combustion. The obtained (calculated) propagation velocity of the kerosene aerosol combustion front, depending on the kerosene mass concentration and the initial temperature of the mixture, is in good agreement with the experimental data.