Numerical investigation of condensation on microstructured surface with wettability patterns

The theoretical equations on heat and mass transfer are set up for laminar free Convection film condensation of vapor–gas mixture. In the theoretical equations only dimensionless wall temperature gradient and condensate mass flow rate parameter are no-given variables respectively for prediction of heat and mass transfer rates. The laminar free Convection film condensation of water vapor in presence of air on a vertical flat plate is taken as example for the numerical solutions on condensate heat and mass transfer, including those on the dimensionless temperature gradient and mass flow rate parameter. Both by decreasing the bulk vapor mass fraction and the reference wall subcooled grade, the wall dimensionless temperature gradient will increase at accelerative pace. Both decreasing the bulk vapor mass fraction and the reference wall subcooled grade will cause decreasing the condensate mass flow rate parameter at accelerative pace. These phenomena demonstrate the decisive effect of the non-condensable gas on condensate heat and mass transfer of the laminar forced film condensation of vapor–gas mixture. The system of the rigorous key solutions on the wall dimensionless temperature gradient and the condensate mass flow rate parameter is formulated to the simple and reliable equations for the laminar free Convection film condensation of water vapor–air mixture. Coupled with these formulated equations, the theoretical equations on the condensate heat and mass transfer can be respectively used for reliable and simple prediction of heat and mass transfer rate on laminar free Convection film condensation of water vapor–air mixture. Additionally, it is found that the condensate heat transfer rate is dominated by the wall subcooled temperature\(t_\text{w}-t_{\text{s},\,\text{int}}\) and the wall temperature gradient, the condensate mass flow rate is dominated by the condensate mass flow rate parameter, and the condensate heat transfer rate is identical to the condensate mass flow rate. Due to the quite different condensate mechanisms, the condensate heat and mass transfer rate of the laminar free Convection film condensation from vapor in presence of non-condensable gas is quite different from that of pure vapor, even for \(C_{\text{mv},\infty } \rightarrow 0.\) KeywordsHeat Transfer RateFilm CondensationCondensate Heat TransferCondensate MassPure VaporThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The mixed Marangoni assisting/resisting flow of a nanofluid with thermal radiative heat flux is analyzed when thermal and solutal buoyant forces are significant. The heat and mass transfer rates are simultaneously optimized by utilizing the Response Surface Methodology (RSM). The face‐centered Central Composite Design (fc‐CCD) is used for the numerical experimental design involved in RSM. The sensitivities of the heat and mass transfer rates are evaluated to compare the impact of the thermal and solutal buoyant forces. Appropriate scaling and similarity transformations are utilized to simplify the problem and then numerical solutions are obtained. The nanoliquid flow, temperature, and concentration profiles are plotted for the buoyancy assisting and opposing Marangoni cases. The Marangoni flow with opposite buoyancy is found to have a greater magnitude of velocity while the flows assisted by the buoyancy have a greater magnitude of temperature and concentration profiles. Thermal buoyancy force has a predominant (0.6%) impact on both heat and mass transfer rates compared to solutal buoyancy force. Buoyancy forces are positively sensitive to heat and mass transfer rates. The thermal radiation aspect augments the temperature profile throughout the domain. The optimized mass and heat transfer rates ( and ) is achieved at the highest level of the buoyancy forces and ratio of Marangoni numbers.

Phase-change lattice Boltzmann simulation of condensate falling and heat transfer on hybrid-wettability surface in the presence of non-condensable gas

Internal cooling is one of the effective techniques to cool turbine blades from inside. This internal cooling is achieved by pumping a relatively cold fluid through the internal‐cooling channels. These channels are fed through short channels placed at the root of the turbine blade, usually called entrance region channels. The entrance region at the root of the turbine blade usually has a different geometry than the internal‐cooling channel of the blade. This study investigates numerically the fluid flow and heat transfer in one‐pass smooth isothermally heated channel using the RNG k − ε model. The effect of Reynolds number on the flow and heat transfer characteristics has been studied for two mass flow rate ratios (1/1 and 1/2) for the same cooling channel. The Reynolds number was varied between 10 000 and 50 000. The study has shown that the cooling channel goes through hydrodynamic and thermal development which necessitates a detailed flow and heat transfer study to evaluate the pressure drop and heat transfer rates. For the case of unbalanced mass flow rate ratio, a maximum difference of 8.9% in the heat transfer rate between the top and bottom surfaces occurs at Re = 10 000 while the total heat transfer rate from both surfaces is the same for the balanced mass flow rate case. The effect of temperature‐dependent property variation showed a small change in the heat transfer rates when all properties were allowed to vary with temperature. However, individual effects can be significant such as the effect of density variation, which resulted in as much as 9.6% reduction in the heat transfer rate.

Wettability plays a vital role in fundamental researches and practical applications. Wettability control and patterning have been widely studied in various fields. Although researches have grown rapidly, the methods are still restricted by limitations including complicated processes, high equipment requirements and shortage of methods to treat complex surfaces. Here we report a simple, low cost, array-based wettability control and patterning method via in-situ modification by flexible plasma stamp. Wettability control and patterning on surfaces of superhydrophobic aluminum, superhydrophobic PDMS and silicon, even plant leaf and fruit are achieved. The relationships between the wettability and the treatment time are investigated. We elucidate that the wetting states can also be reversible. The surface modification mechanism of in-situ plasma treatment is further investigated. Utilizing the step by step treatment, gradient and arbitrary wettability patterning on surfaces have been obtained. Notably, the patterned wettability on the inner surface of a tube has been realized, which has never been reported. Finally, in-situ wettability patterning is applied to achieve microfluidics channels on the inner surface of superhydrophobic tube. This work will bring new insights into the study of wetting field and stimulate more applications on wettability control and patterning.

In this paper, we report steady state experimental data for laminar condensation heat transfer on bank of smooth horizontal tubes for fluids with Stefan number greater than unity. The condensation experiments were carried out in saturated vapor of FC5311 ® on 9.5 mm, 12.7 mm, 15.88 mm and 22.2 mm diameter copper tubes for spacing-to-diameter ratios (s/D) ranging from 2 to 12. Experimental heat transfer coefficients were calculated for each tube based on the sensible heat gain of the cooling water. A comparison with the past analytical predictions of the ratio of heat transfer from the top tube to that from the lower tubes in a bank of tubes reveals two major differences. First, Chen’s correlation over-predicts heat transfer rates from the lower tubes. Second, our data indicate that the heat transfer rate from the lower tubes is a strong function of s/D. Assumptions made in Chen’s analysis are critically examined and the discrepancy is explained in terms of non-validity of those assumptions and ignoring of the different hydrodynamics that arise for different vertical spacing between the tubes. The condensate and the momentum gain by the jets issuing from the upper tubes are different for different s/D ratios. For low spacing-to-diameter ratio (s/D=6 ) tube banks, condensation heat transfer from the lower tubes deteriorates due to the formation of a finite condensate film on the top of these tubes. For high spacing-todiameter ratio (s/D>6) tube banks, however, momentum gain between tubes leads to instability of condensate jets thereby enhancing the condensation heat transfer on the lower tubes. Finally, heat transfer correlations are formulated to predict condensation heat transfer for tube banks with different spacing-todiameter ratios.

Interfacial heat transfer during subcooled flow boiling

Numerical investigation of the gravity effect on two-phase flow and heat transfer of neon condensation inside horizontal tubes

The buoyancy-driven transient double-diffusive convection in a square cavity filled with water-saturated porous medium is studied numerically. While the right and left side wall temperatures vary linearly from θa to θo and θo to θb, respectively, with height, the top and bottom walls of the cavity are thermally insulated. The species concentration levels at the right and left walls are c1 and c2, respectively, with c1>c2. The Brinkman–Forchheimer extended Darcy model is considered to investigate the average heat and mass transfer rates and to study the effects of maximum density, the Grashof number, the Schmidt number, porosity, and the Darcy number on buoyancy-induced flow and heat transfer. The finite volume method with power law scheme for convection and diffusion terms is used to discretize the governing equations for momentum, energy, and concentration, which are solved by Gauss–Seidel and successive over-relaxation methods. The heat and mass transfer in the steady-state are discussed for various physical conditions. For the first time in the literature, the study of transition from stationary to steady-state shows the existence of an overshooting between the two cells and in the average Nusselt number. The results obtained in the steady-state regime are presented in the form of streamlines, isotherms, and isoconcentration lines for various values of Grashof number, Schmidt number, porosity and Darcy number, and midheight velocity profiles. It is found that the effect of maximum density is to slow down the natural convection and reduce the average heat transfer and species diffusion. The strength of convection and heat transfer rate becomes weak due to more flow restriction in the porous medium for small porosity.

Numerical study on the condensation flow and heat transfer characteristics of hydrocarbon mixtures inside the tubes of liquefied natural gas coil-wound heat exchangers

Condensation is a universal phenomenon that occurs in nature and industry. Previous studies have used superhydrophobicity and liquid infusion to enable superior liquid repellency due to reduced contact angle hysteresis. However, small condensate droplets remain immobile on condensing surfaces until they grow to the departing size at which the body force can overcome the contact line pinning force. Hence, condensation heat transfer is limited by these remaining droplets that act as thermal barriers. To break these limitations, we introduce vibrational actuation to a slippery liquid-infused nanoporous surface (SLIPS) and show enhanced droplet mobility, controllable condensate repellency, and more efficient heat transfer compared to static SLIPSs. We demonstrate 39% smaller departing droplet size and 8× faster droplet departing speeds on the dynamic vibrating SLIPS compared to the nonactuated SLIPS. To understand the implications of these behaviors on heat transfer, we investigate the condensate area coverage and droplet distribution to verify enhanced dewetting on dynamic vibrating SLIPSs. Using well-validated heat transfer models, we demonstrate enhanced condensation heat transfer on dynamic SLIPSs due to the higher population of smaller condensate droplets (<100 μm). In addition to condensation heat transfer, we also show that vibrating SLIPSs can enhance droplet collection. This work utilizes the synergistic combination of surface chemistry and mechanical actuation to realize enhanced droplet mobility and heat transfer in an electrically controllable and switchable manner.

Lattice Boltzmann simulation of dropwise condensation on the microstructured surfaces with different wettability and morphologies

Investigating condensation from humid air under mixed convection regime

The numerous studies on condensation flow patterns and heat transfer were focused on the horizontal inside single tube. A number of heat and mass transfer correlations are used for design of shellside condensers based on tubeside condensation flow regimes. Due to a complex geometry and measurement difficulty in a tube bundle, there are few publications reported on shellside condensation flow regime and heat transfer characteristics. To investigate the condensation flow patterns and heat and mass transfer mechanism at the different flow regimes, a horizontal shellside condenser was tested from a multipurpose condensation rig recently. The horizontal test bundle is made of 36 tubes with the staggered tube layout. The tube OD is 19 mm and the tube length is 1.0 m using stainless steel. Four visualization windows were placed on the front and back sides on the shell for photographing condensation flow patterns. Steam and steam/air mixture were used as the test fluids. The condensation flow patterns, condensate film thickness and droplets distribution were recorded using a high-speed digital camera at a wide range of condensation process conditions. The experimental data show that the condensation flow regime changes from the shear-controlled flow to gravity-controlled flow depending on the vapor and condensate loads, bundle location and the concentration of the non-condensable gas. These experimental data provide a fundamental approach for developing the heat and mass transfer correlateons at different shellside condensation patterns. This paper presents the experimental result on shellside condensation patterns associated with heat transfer characteristics.

Forced convective condensation flow and heat transfer characteristics of hydrocarbon mixtures refrigerant in helically coiled tubes