Condensation heat transfer of R245fa in horizontal copper pipe is studied in this paper. In the experimental study, the pipe inner diameter is 4.38 mm, the condensing temperature range from 50∼60°C, mass flow rate ranges from 3.132kg/h to 11.988kg/h, and inlet ranges from 0.89∼1. The research results indicate that the condensation coefficient is rose with the condensing; and it will rise when the inlet quality ranges from 0.89 to 1. Experimental value and the existing two-phase flow correlation are compared in this paper. It shows that the condensation heat transfer coefficient predicted by Haraguchi correlation has a high accuracy relatively. The research provides a foundation to promote development of microchannel high efficiency heat exchanger and it is useful to practical engineering.

Water vapor condensing process inside horizontal tube has been studied in the paper. A CFD simulation model has been developed with Fluent software. An experimental investigation has also been conducted with an 8-mm inside diameter horizontal tube. As well as the heat transfer correlations available in literatures, the CFD model has been validated against the experimental results. The discrepancy of the simulation model is within 15%. The effect of wall temperature, inlet vapor velocity and inlet superheating degree on local heat transfer coefficient has been analyzed. The simulation model can be employed in future work to develop a condensation heat transfer correlation in horizontal tube.

For the state of condensation in tube, liquid condensate separation in middle process can prolong the state of steam entrance region of higher heat transfer coefficient. It is called short-tube effect theory. Combined with the traditional condenser, a shell and tube condenser was designed for experiment research in this paper, and compared with the traditional condenser by opening liquid distribution pipes arranged in both sides of condenser. The results showed that liquid distribution pipes with different diameter have different condensation effect. Under the same steam flow rate of inlet, liquid distribution pipes with different combination of diameter and number indicated that its coefficient of heat transfer are higher than the traditional heat transfer by 14.2%, 15.5% and 25.1%. This result illustrated that heat exchange efficiency of a shell and tube condenser with liquid distribution pipes is better than a traditional condenser.

Understanding the condensation mechanism is crucial to enhance the heat transfer performance of numerous industrial applications such as power generations, fog harvesting, water desalination, cooling of nuclear reactor, and thermal management of electronic device. In the present study, simulations are performed to investigate the effect of surface wettability on droplet growth dynamics during dropwise condensation. To simulate droplet growth dynamics involving phase change heat transfer, thermal lattice Boltzmann method has been employed with two distribution function for fluid and temperature field. Simulations performed in this work are used to analyze the effect of surface wettability on nucleation time and the evolution of average droplet radius, height, base diameter, and contact angle of the droplet. It is observed that nucleation time increases exponentially with the contact angle. The growth rate of droplet is higher for smaller droplets compared to larger droplets.

Jumping-droplet enhanced condensation has recently attracted huge interest due to its remarkable potential of heat transfer performance enhancement, and studies have been done to design superhydrophobic surfaces with various surface morphologies. We fabricated a superhydrophobic micromesh-covered surface using a facile and scalable method. ESEM condensation experiment results show that droplets in pores formed by the mesh wires had faster growth rate in the upward direction than droplets on wires. This is mainly because of the confining role of the wires and higher heat transfer rate due to larger solid-liquid contact area. Also, these droplets always jumped at the size of pores (∼35 μm) when they coalesced with other droplets on wires. Moreover, droplets in pores were distorted by mesh wires, resulting in larger surface area. Theoretical predictions show, for a specific droplet radius, coalescence jumping of distorted droplets on the mesh-covered surface releases more excess surface free energy, and has larger jumping velocity than that of spherical droplets on the plate surface without mesh. This better performance was further validated by constant exposure of those two surfaces to electron beam during which work of adhesion was gradually increased. As expected, droplets on the mesh-covered surface coalesced and jumped while coalescing droplets on the plate surface could not as the exposure time increased. Our results offer new insights for designing hierarchical structured superhydrophobic surfaces to further enhance the performance of condensation heat transfer processes.

An approach to design multi-channel cylinder dryer was proposed. The heat transfer performance and flow characteristic under various structural parameters were analyzed. First, an experiment was designed and set up to measure the condensing heat transfer coefficient and the pressure drop in order to verify the applicability of the Cavallini’s correlation. Then, the relationship among the count of channels, aspect ratio, spacing ratio, width, height and hydraulic diameter of a channel was given. Finally, the correlation of condensing heat transfer and the homogeneous model were introduced in order to observe the heat transfer performance and flow characteristic of the multi-channel cylinder dryer affected by different structures. The study reveals that the structural parameters including count of channels, aspect ratio, spacing ratio of a channel dramatically influence the condensation heat transfer coefficient and frictional resistance of the steam. Based on the selected paper machine, it is suggested that the overall performance of the multi-channel cylinder dryer is best if the count of channels is 150–200, the aspect ratio is 1:3 and the spacing ratio is 1:1–1:3.

Despite that using surface-roughness-induced superhydrophobic surface as a solution for ice/snow accretion issues has achieved extensive progresses, its icephobicity breaks down in case of condensation frosting, while the high aspect ratio structure brings more concerns on its durability and sustainability. In this work we investigated condensate frosting on substrates fabricated with patterned micropillars having a small aspect ratio, and studied the freezing propagation with different pattern sizes. The results show that a coarse patterned substrate can effectively suppress the freeing propagation while a fine patterned one can drastically promote the freezing propagation. Frost coverage can also be reduced with proper pattern design. A theoretical model was developed to explain the mechanism of surface ice propagation, and agrees well in tendency with experiment measurements. The aim of this study is to provide some new insights on the influence of surface morphology on ice growth.

In this article, the heat transferring property of the copper-water nanofluids in self-exciting mode oscillating flow heat pipe under different laser heating power is experimented, as well as is compared with that of the distilled water medium in self-exciting mode oscillating flow heat pipe under same heating condition. The objective of this article is to provide the heat transfer characteristics of Cu-H 2 O nanofluids in self-exciting mode oscillating-flow heat pipe under different laser heating input, and to compare with the heat transfer characteristics of the same heat pipe with distilled water as working fluids. The SEMOS HP used in this experiment is made of brass tube with 2mm interior diameter, which is consisted of 8 straight tubes with 4 turns’ evaporation section and 12 turns’ condensation section. The heat resource for the evaporation zone is eight channel quantum pitfall diode array semi-conductor laser heater with 940nm radiation wave length, while the radiation power of each channel is changeable within 0–50W and the facular size is 1×30mm 2 . The condensation section is set in a cooling water tank in which water is from another higher tank. The actual transferring rate could be calculated by the flow rate of the cooling water and the change of the temperature. The change of the temperature of the heat pipe wall is measured by those thermo-couple fixed in different section in the heat pipe and data is collected by a data acquisition. In the heat pipe the fluid filling rate is 43%, the pressure is 2.5×10 −3 Pa, and the heat pipe inclination angle is 55° while the size of the brass particle in the nanofluids is less than 60nm and volume proportion is 0.5%. In this paper, the particularity of heat transfer rate of the SEMOS heat pipe with Cu-H 2 O fluid has been experimentally confirmed by changing the proportion of working fluid and Cu nonsocial particles in the heat pipe. By comparing the experimental result of these two different medium in the SEMOS HP, it is shown that the heat transferring rate with brass-water nanofluids as medium is much better than that with distilled water as medium under same volume proportion.

Surface with hierarchical structures to enhance heat transfer performance has been investigated for decades. Here we present a unique two-stage vaporization phenomenon observed when the heated surface is covered to confine the thickness of liquid film. This multi-stage vaporization will be critical for optimization applications in thermal management. The hierarchical surface is prepared using: a) isotropic etching to form micro-pillars, and b) wet-chemical etching to produce silicon nanowires (SiNWs) on sidewalls of micro-pillars. Oxygen plasma was applied to modify their surface wettability from superhydrophobic to superhydrophilic. A glass slide was placed right on top of the structures, covering about half of the heated surface, and mechanically clamped by a stainless-steel block. Therefore, the liquid film is about the same height as the micro-pillars. The hierarchical surface was wetted with deionized water before being heated. There are two clear and separate stages that were observed under an high-speed camera: i) thinning while liquid is vaporizing, and ii) mixture of vaporizing and condensation until total dry-out happens. The detailed dynamics of this two-stage vaporization were explored and reported in this study.

It is well known that a phase transition from liquid to vapor occurs in the thermal boundary layer adjacent to a nanoparticle that has a high temperature upon irradiation with a high-power laser. In this study, the mechanism by which the evaporated layer adjacent to a laser-irradiated nanoparticle can grow as a bubble was investigated. The pressure of the evaporated liquid volume due to heat diffusion from the irradiated nanoparticle was estimated using a bubble nucleation model based on molecular interactions. The bubble wall motion was obtained using the Keller-Miksis equation. The density and temperature inside the bubble were obtained by solving the continuity and energy equations for the vapor inside the bubble. The evaporation of water molecules or condensation of water vapor at the vapor-liquid interface and the homogeneous nucleation of vapor were also considered. The calculated bubble radius -time curve for the bubble formed on the surface of a gold particle with a diameter of 9 nm is close to the experimental result. Our study reveals that an appropriate size of the evaporated liquid volume and a large expansion velocity are important parameters for the formation of a transient nano-sized bubble. The calculation result suggests that homogeneous condensation of vapor rather than condensation at the interface occurs.

Vapor condensation on silicon nanotubes has been simulated by classical molecular dynamics to understand how the nucleation and condensation process for pores is affected. Two different nanotube aspect ratios were examined to see if there are growth rate changes. The rate for the two different types of nanotubes did not show significant variation meaning that the aspect ratio is an insignificant factor to enhance condensation. This result is consistent with previous nanorod studies. The supersaturated vapor gathered both inside and outside of the tube. Unlike the growth rate, however, the occurrence of homogeneous nucleation was hindered contrary to other basic geometries in previous studies.

Mixed Refrigerant Joule Thomson (MR-JT) refrigerators are widely used in various kinds of cryogenic systems these days. Although heat transfer coefficient estimation for a multiphase and multi-component fluid in cryogenic temperature range is necessarily required in the heat exchanger design of MR-JT refrigerator, it has been rarely discussed so far. In this paper, condensation and evaporation heat transfer coefficients of mixed refrigerant are measured in a microchannel heat exchanger. Printed Circuit Heat Exchanger (PCHE) has been developed as a compact microchannel heat exchanger and used in the experiment. Several two-phase heat transfer coefficient correlations are examined to discuss the experimental measurement results. The result of this paper shows that cryogenic mixed refrigerant heat transfer coefficients can be estimated by conventional two-phase heat transfer coefficient correlations.

Detailed experimental investigations of condensation in microchannels where local heat flux and surface temperature were measured along the channel are compared with theoretical results for the special case of annular, laminar flow. The theoretical model includes surface tension driven transverse flow towards the corners of the channel as well as shear stress driven streamwise flow in an otherwise Nusselt treatment. The theory has no empirical input. When distributions along the channel of the local vapor and wall temperatures are given, local heat flux and heat-transfer coefficient, as well as local vapor quality, may be calculated. Such detailed experimental data have only recently become available. Strict implementation of the theory requires that the onset of condensation occurs within the channel, i.e. the vapor is saturated or superheated at the inlet. The comparisons show remarkably good agreement with the experimental data for two fluids and covering a wide range of experimental conditions.

Condensation is an important process in both emerging and traditional power generation and water desalination technologies. Superhydrophobic nanostructures promise enhanced condensation heat transfer by reducing the characteristic size of departing droplets via a surface-tension-driven mechanism [1]. In this work, we investigated a scalable synthesis technique to produce oxide nanostructures on copper surfaces capable of sustaining superhydrophobic condensation and characterized the growth and departure behavior of condensed droplets. Nanostructured copper oxide (CuO) films were formed via chemical oxidation in an alkaline solution. A dense array of sharp CuO nanostructures with characteristic heights and widths of ∼1 μm and ∼300 nm, respectively, were formed. A gold film was deposited on the surface and functionalized with a self-assembled monolayer to make the surfaces hydrophobic. Condensation on these surfaces was then characterized using optical microscopy (OM) and environmental scanning electron microscopy (ESEM) to quantify the distribution of nucleation sites and elucidate the growth behavior of individual droplets with a characteristic size of ∼1 to 10 μm at low supersaturations. Comparison of the observed behavior to a recently developed model for condensation on superhydrophobic surfaces [2, 3] suggests a restricted regime of heat transfer enhancement compared to a corresponding smooth hydrophobic surface due to the large apparent contact angles demonstrated by the CuO surface.

Bio-oil, produced from biomass feedstock like rice hull, straw, wood flour and other biomass wastes by fast pyrolysis where the biomass feedstock is heated and pyrolized with a rapid rate and the pyrolysis gases produced are condensed rapidly, is an interesting potential alternative fuel oil. Cooling rate of the biomass pyrolysis gas is an important factor effecting the production of bio-oil. In order to speed up the cooling rate, the high temperature biomass pyrolysis gas is cooled and condensed by spray droplets of produced bio-oil with low temperature in this paper. It was assumed that the chemical reactions among the components of pyrolysis gas can be ignored, a theoretical model based on the classic film model and the Maxwell-Stefan equation was presented to simulate the heat and mass transfer characteristics of the spray condensation of biomass pyrolysis gas. The effects of the initial pyrolysis gas temperatures, the initial bio-oil droplets temperatures and diameters, and the flow ration of the gas and the liquid droplets on the heat and mass transfer between the gas and the liquid droplets were analyzed by the model.

Condensation on superhydrophobic nanostructured surfaces offers new opportunities for enhanced energy conversion, efficient water harvesting, and high performance thermal management. Such surfaces are designed to be Cassie stable, which minimize contact line pinning and allow for passive shedding of condensed water droplets at sizes smaller than the capillary length. In this work, we investigated in situ water condensation on superhydrophobic nanostructured surfaces using environmental scanning electron microscopy (ESEM). The “Cassie stable” surfaces consisted of silane coated silicon nanopillars with diameters of 300 nm, heights of 6.1 μm, and spacings of 2 μm, but allowed droplets of distinct suspended (S) and partially wetting (PW) morphologies to coexist. With these experiments combined with thermal modeling of droplet behavior, the importance of initial growth rates and droplet morphology on heat transfer is elucidated. The effect of wetting morphology on heat transfer enhancement is highlighted with observed 6× higher initial growth rate of PW droplets compared to S droplets. Consequently, the heat transfer of the PW droplet is 4–6× higher than that of the S droplet. To compare the heat transfer enhancement, PW and S droplet heat transfer rates are compared to that of a flat superhydrophobic silane coated surface, showing a 56% enhancement for the PW morphology, and 71% degradation for the S morphology. This study provides insight into importance of local wetting morphology on droplet growth rate during superhydrophobic condensation, as well as the importance of designing CB stable surfaces with PW droplet morphologies to achieve enhanced heat transfer during dropwise condensation.

The paper examines the special case of annular laminar flow pressure drop, or more precisely pressure gradient, during condensation in microchannels. This is the only flow regime permitting wholly theoretical solution without having recourse to experimental data. Solutions are obtained and comparisons made with empirical formulae for void fraction (needed to calculate the momentum pressure gradient) when obtaining the friction pressure gradient from experimentally measured or “total” pressure gradient. To date calculations and comparisons are restricted to one fluid (R134a), one channel section and one flow condition. For the case considered it is found that earlier approximate models for estimating void fraction agree quite well with the theoretical annular flow solutions. There is, however, significant difference between momentum pressure gradients obtained from approximate models used in the earlier investigations and that given by the theoretical annular flow solution which is (numerically) higher than all of them. The annular flow solution indicates that the momentum pressure gradient is not small in comparison with the friction pressure gradient. The friction pressure gradient in the annular flow case is appreciably smaller than given by the earlier correlations.

The clustering phenomenon on the solid wall during dropwise condensation is analyzed with reflection spectrum. From the theoretical prediction of the reflectivity for thin liquid films with different thickness on stainless steel surface, it is ascertained that the reflectivity corresponds to the coacervate characteristics of the steam molecular. Furthermore, the experimental data of the reflection spectrum during dropwise condensation in literature also demonstrated that the reflection feature and so as the coacervate characters lie between liquid and steam after the droplet departing during an actual continuous condensation process. The clustering model is used to analyze the results, indicating that clusters form on the blank surface. And it is found that different microstructures of the solid wall would lead to different deposition rates of the clusters, which prompts an effective way to enhance heat transfer process of condensation by accelerating the deposition rate of clusters with surface modification.

Evaporative cooling, which exploits the large latent heats associated with phase change, is of interest in a variety of thermal management technologies. Yet our fundamental understanding of thermal and mass transport remains limited. Evaporation and condensation can change the local temperature, and hence surface tension, along a liquid-vapor interface. The resulting thermocapillary stresses are dominant at small length scales in many cases. For the vast majority of single-component coolants, surface tension decreases as temperature increases, resulting in thermocapillary stresses that drive the liquid away from hot regions, leading to dryout, for example. The direction of flow driven by thermocapillary stresses is therefore consistent with that driven by buoyancy effects due to changes in the liquid density with temperature. However, a number of binary “self-rewetting fluids,” consisting of water-alcohol mixtures, have surface tensions that increase with temperature, leading to thermocapillary stresses that drive liquid towards hot regions, improving cooling performance. Although not all binary coolants are self-rewetting, all such coolants are subject to solutocapillary stresses, where differential evaporation of the two fluid components leads to changes in local species concentration at the liquid-vapor interface, and hence in surface tension. Given the lack of general models of thermal and mass transport in nonisothermal two-phase flows, experimental studies of convection in simple fluids and binary alcohol-water mixtures due to evaporation and condensation driven by a horizontal temperature gradient were performed. In these initial studies, both the simple and binary fluids have thermocapillary stresses that drive liquid away from hot regions. However, the binary fluid also has solutocapillary stresses that drive liquid towards hot regions. Particle-image velocimetry (PIV) is used to nonintrusively measure the velocity and temperature fields in a layer of liquid a few mm in depth in a 1 cm × 1 cm × 4.85 cm sealed and evacuated cuvette heated on one end and cooled on the other end.

Rayleigh-Bénard and Marangoni convection in a layer of a homogeneous fluid with a free surface in the absence of phase change is a classic (and extensively studied) problem of fluid mechanics. Phase change has a major effect on the convection problem. Most notably, significant latent heat generated at the free surface as a result of phase change can dramatically alter the interfacial temperature, and hence, the thermocapillary stresses. Furthermore, differential evaporation in binary fluids can lead to considerable variation in the concentration field, producing solutocapillarity stresses, which can compete with thermocapillarity and buoyancy. This talk describes numerical studies of convection in alcohol and alcohol-water mixtures due to a horizontal temperature gradient in the presence of phase change. We illustrate how the composition of the liquid and the presence of non-condensable gases (e.g., air) can be used to alter the balance of the dominant forces. In particular, by adding or removing air from the test cell, the direction of the flow can be reversed by emphasizing either the thermocapillary or the solutocapillary stresses.