Enthalpy Transfer Coefficient Research Articles

Determining heat and mass exchange kinetic in a column heat exchanger with direct phases contact

The object of this study was the processes of heat and mass transfer in a column heat exchanger with direct phases contact. The investigated problem is related to the need to determine the estimates of the kinetic characteristics of heat exchange during the concentration of solutions in a contact heat exchanger equipped with dual-flow trays. In particular, it was assumed that the determination of the influence of the gas velocity in the apparatus and the density of liquid irrigation of the plates, as well as the geometry of the plates, on the kinetic coefficients would make it possible to reveal the patterns of heat and mass exchange between the liquid and the air in contact with it in the column apparatus. It was determined that to increase the intensity of mass transfer in the gas and heat transfer in the liquid, it is necessary to increase the gas velocity and irrigation density. Then the gas velocity and irrigation density have approximately the same effect on the intensity of mass transfer in the gas and heat transfer in the liquid. When studying the effect of the geometry of the plate on the kinetic coefficients of heat transfer in the liquid and mass transfer in the gas, it was established that the value of the portion of the free cross section of the plate has a decisive influence on the value of the considered kinetic coefficients. A generalized equation was built, which makes it possible not only to calculate the enthalpy transfer coefficient during the interaction of sodium chloride solution with air but also to determine the limiting stage of this process. The results could be used to design a unit for concentrating a hot solution of sodium chloride by evaporating water during blowing with air in a contact heat exchanger. This would make it possible to obtain crystalline sodium chloride using secondary energy resources and other non-traditional sources of thermal energy

Design method for heating towers used in vapor-compression heat pump systems

Given that they are highly efficient in summer and frost-free in winter, heating tower heat pumps are evolving rapidly. However, lack of an appropriate design method of heating towers is complicating the design process and causing a mismatch between cooling and heating conditions. To address this problem, this study develops a total heat transfer model taking enthalpy difference as the driving force, then experiments are carried out to test the volumetric enthalpy transfer coefficients. Based on the model and the coefficients, a novel design method is proposed based on curves of required and available transfer units, involving four steps, namely: 1) determination of design conditions, 2) calculating tower thermal performance, 3) sizing x-y section of packing, 4) determination of air flow rate and packing size. Finally, a performance comparison and matching degree analysis of the designed tower are presented. The results show that the proposed design method is simple and clear by avoiding trial-and-error. The designed tower saves considerable power consumption of the tower fans and pumps and achieves good matching degree between its heating and cooling capacities by setting reasonable approaches and ranges. The model, the coefficients and the design method can be used accurately by researchers/designers.

Investigating the impact of atmosphere–wave–ocean interactions on a Mediterranean tropical-like cyclone

Understanding the governing mechanisms of atmosphere–wave–ocean​ interactions is critical for unravelling the formation and evolution mechanisms of severe weather phenomena. This study aims at investigating the effects of atmosphere–wave–ocean​ feedbacks on a Mediterranean tropical-like cyclone (medicane), occurred on 27–30 September 2018 at the central-eastern Mediterranean Sea and characterized by severe environmental and socioeconomic impact. To unveil the interactions across the air–sea interface, the medicane was simulated by an integrated modelling system consisting of the Chemical Hydrological Atmospheric Ocean wave System (CHAOS), upgraded by embedding to it the Nucleus for European Modelling of the Ocean (NEMO) as ocean circulation component. Coupled simulations revealed that air–seaheat transfer and Ekman pumping, bringing sub-surface cold waters in upper ocean layers (upwelling), caused SST cooling (∼2–3 °C). SST cooling triggered a negative feedback loop procedure tending to balance between atmospheric and ocean processes. It also attenuated the cyclone and, subsequently, reduced the atmospheric energy embedded in ocean through the upper ocean vertical stratification weakening, thus, upper ocean vertical mixing, upwelling and SST cooling. The waves adjusted this feedback loop making the system more resistant in air–sea flux variations. Waves additionally weakened the cyclone not only due to the kinetic energy loss in the lower-atmosphere but also due to the enhancement of SST cooling which is attributed to the strengthening of Ekman pumping and vertical mixing, forced by wind stress increase. Nevertheless, waves partially balanced the air–wave–sea exchanges through the slight enthalpy flux gain under high wind conditions which is explained by considering the increase of enthalpy transfer coefficient in rougher sea areas.

Momentum and heat transfer across the foam-covered air-sea interface in hurricanes

Procedures of formal averaging over the air-sea interface are applied to both momentum and enthalpy surface-transfer coefficients, CD and CK, in hurricane conditions. The transfer coefficients across the total area of the sea surface and water-covered portions of the sea surface, CD, CK, and CDw, CKw, are estimated by measurements in the open-sea and laboratory (foam-free) conditions, respectively, while the transfer coefficients across the foam-covered portion of the sea surface, CDf, Kf, which cannot be measured directly, are estimated using the splitting relations. Applying the Monin-Obukhov similarity theory at the neutral stability atmospheric conditions to the transfer coefficients (separately for the foam-free, foam-covered, and total sea surfaces) yields the roughness lengths ZDw, ZKw, ZDf, ZKf, and ZD, ZK. The study is aimed at the description of an anomalous, as compared with laboratory conditions, behavior of the momentum and enthalpy transfer coefficients in hurricane conditions with wind speed U10 by the effect of the foam slipping layer sandwiched between the atmosphere and the ocean.

Effects of horizontal resolution and air–sea flux parameterization on the intensity and structure of simulated Typhoon Haiyan (2013)

Abstract. This study investigates the effects of horizontal resolution and surface flux formulas on typhoon intensity and structure simulations through the case study of the Super Typhoon Haiyan (2013). Three sets of surface flux formulas in the Weather Research and Forecasting Model were tested using grid spacings of 1, 3, and 6 km. Increased resolution and more reasonable surface flux formulas can both improve typhoon intensity simulation, but their effects on storm structures differ. A combination of a decrease in momentum transfer coefficient and an increase in enthalpy transfer coefficients has greater potential to yield a stronger storm. This positive effect of more reasonable surface flux formulas can be efficiently enhanced when the grid spacing is appropriately reduced to yield an intense and contracted eyewall structure. As the resolution increases, the eyewall becomes more upright and contracts inward. The size of updraft cores in the eyewall shrinks, and the region of downdraft increases; both updraft and downdraft become more intense. As a result, the enhanced convective cores within the eyewall are driven by more intense updrafts within a rather small fraction of the spatial area. This contraction of the eyewall is associated with an upper-level warming process, which may be partly attributed to air detrained from the intense convective cores. This resolution dependence of spatial scale of updrafts is related to the model effective resolution as determined by grid spacing.

Direct numerical simulation of turbulent heat transfer across a sheared wind-driven gas–liquid interface

Turbulent heat transfer across a sheared wind-driven gas–liquid interface is investigated by means of a direct numerical simulation of gas–liquid two-phase turbulent flows under non-breaking wave conditions. The wind-driven wavy gas–liquid interface is captured using the arbitrary Lagrangian–Eulerian method with boundary-fitted coordinates on moving grids, and the temperature fields on both the gas and liquid sides, and the humidity field on the gas side are solved. The results show that although the distributions of the total, latent, sensible and radiative heat fluxes at the gas–liquid interface exhibit streak features such that low-heat-flux regions correspond to both low-streamwise-velocity regions on the gas side and high-streamwise-velocity regions on the liquid side, the similarity between the heat-flux streak and velocity streak on the gas side is more significant than that on the liquid side. This means that, under the condition of a fully developed wind-driven turbulent field on both the gas and liquid sides, the heat transfer across the sheared wind-driven gas–liquid interface is strongly affected by the turbulent eddies on the gas side, rather than by the turbulent eddies and Langmuir circulations on the liquid side. This trend is quite different from that of the mass transfer (i.e. $\text{CO}_{2}$ gas). This is because the resistance to heat transfer is normally lower than the resistance to mass transfer on the liquid side, and therefore the heat transfer is controlled by the turbulent eddies on the gas side. It is also verified that the predicted total heat, latent heat, sensible heat and enthalpy transfer coefficients agree well with previously measured values in both laboratory and field experiments. To estimate the heat transfer coefficients on both the gas and liquid sides, the surface divergence could be a useful parameter, even when Langmuir circulations exist.

Enthalpy Transfer across the Air–Water Interface in High Winds Including Spray

Abstract Controlled experiments were conducted in the Air–Sea Interaction Saltwater Tank (ASIST) at the University of Miami to investigate air–sea moist enthalpy transfer rates under various wind speeds (range of 0.6–39 m s−1 scaled to equivalent 10-m neutral winds) and water–air temperature differences (range of 1.3°–9.2°C). An indirect calorimetric (heat content budget) measurement technique yielded accurate determinations of moist enthalpy flux over the full range of wind speeds. These winds included conditions with significant spray generation, the concentrations of which were of the same order as field observations. The moist enthalpy exchange coefficient so measured included a contribution from cooled reentrant spray and therefore serves as an upper limit for the interfacial transfer of enthalpy. An unknown quantity of spray was also observed to exit the tank without evaporating. By invoking an air volume enthalpy budget it was determined that the potential contribution of this exiting spray over an unbounded water volume was up to 28%. These two limits bound the total enthalpy transfer coefficient including spray-mediated transfers.

Fallacies of the Enthalpy Transfer Coefficient over the Ocean in High Winds

Abstract Mesoscale and large-scale atmospheric models use a bulk surface flux algorithm to compute the turbulent flux boundary conditions at the bottom of the atmosphere from modeled mean meteorological quantities such as wind speed, temperature, and humidity. This study, on the other hand, uses a state-of-the-art bulk air–sea flux algorithm in stand-alone mode to compute the surface fluxes of momentum, sensible and latent heat, and enthalpy for a wide range of typical (though randomly generated) meteorological conditions over the open ocean. The flux algorithm treats both interfacial transfer (controlled by molecular processes right at the air–sea interface) and transfer mediated by sea spray. Because these two transfer routes obey different scaling laws, neutral-stability, 10-m transfer coefficients for enthalpy CKN10, latent heat CEN10, and sensible heat CHN10 are quite varied when calculated from the artificial flux data under the assumption of only interfacial transfer—the assumption in almost all analyses of measured air–sea fluxes. That variability increases with wind speed because of increasing spray-mediated transfer and also depends on surface temperature and atmospheric stratification. The analysis thereby reveals as fallacious several assumptions that are common in air–sea interaction research—especially in high winds. For instance, CKN10, CEN10, and CHN10 are not constants; they are not even single-valued functions of wind speed, nor must they increase monotonically with wind speed if spray-mediated transfer is important. Moreover, the ratio CKN10/CDN10, where CDN10 is the neutral-stability, 10-m drag coefficient, does not need to be greater than 0.75 at all wind speeds, as many have inferred from Emanuel’s seminal paper in this journal. Data from the literature and from the Coupled Boundary Layers and Air–Sea Transfer (CBLAST) hurricane experiment tend to corroborate these results.

Effects of Sea Spray on Tropical Cyclone Intensity

The intensity of tropical cyclones is sensitive to the rates at which enthalpy and momentum are transferred between sea and air in the high-wind core of the storm. Present models of the wind dependence of these transfer rates suggest that the effective drag coefficient is more than twice the effective enthalpy transfer coefficient at wind speeds above 25 m s−1. Using this ratio in numerical models, however, makes it impossible to sustain storms of greater than marginal hurricane intensity. Some other physical process must, therefore, enhance enthalpy transfer at very high wind speeds. This paper suggests that re-entrant sea spray explains this enhanced transfer. When a spray droplet is ejected from the ocean, it remains airborne long enough to cool to a temperature below the local air temperature but not long enough to evaporate an appreciable fraction of its mass. The spray droplet thus gives up sensible heat and returns to the sea before it has time to extract back from the atmosphere the heat necessary to continue its evaporation. Microphysical modeling, combined with data from the Humidity Exchange over the Sea Experiment (HEXOS), makes it possible to derive an expression for the net enthalpy transfer of re-entrant spray. This spray enthalpy flux is roughly cubic in wind speed. When this relation is used in a numerical simulation of a hurricane, the spray more than compensates for the observed increase in the ratio of drag and enthalpy transfer coefficients with wind speed. The momentum flux associated with sea spray is an important energy sink that moderates the effects of this spray enthalpy flux. Including a parameterization for this momentum sink along with wave drag and spray enthalpy transfer in the hurricane simulation produces results that are similar to ones based on equal transfer coefficients.

A semi-empirical transient method for modelling frost formation on a flat plate

This paper models the frost formation process employing a semi-empirical transient model for a flat plate under forced convection conditions. This continuous differential model employs the Euler's method to solve for the temperature and thickness of the frost layer using existing correlations for the convective heat transfer coefficient and the Lewis analogy to compute both a mass transfer coefficient and an enthalpy transfer coefficient. The model is also based on empirical correlations for predicting the frost thermal conductivity and density. Model results are compared with existing experimental data and with numerical data of other investigators and are found to agree very well in the applicable temperature and humidity range of the frost density and conductivity correlations.

Overall enthalpy transfer coefficient in bubble cooling for heated water stream in an open channel

Overall enthalpy transfer coefficient in bubble cooling for heated water stream in an open channel

An analysis of crossflow cooling towers. The change in the water flow rate is considered.

A theoretical analysis of crossflow cooling tower was studied under conditions such that the change in the water flow rate due to vaporization is not negligible throughout the apparatus and a method of numerical calculation to estimate both the enthalpy of air and temperature of water in a tower was derived, and by applying this method, a calculation method of overall volumetric enthalpy transfer coefficients and the effect of operating conditions on the enthalpy arrival factor, which expresses the cooling effect, were investigated. In addition, the deviation in the tower volume between the case of taking into account and not taking into account the change in the water flow rate in crossflow cooling towers was discussed under the same operating conditions.

Heat and mass transfer in the presence of an inert gas

A new graphical method for solving enthalpy transfer problems is described in which a modified enthalpy is the transfer potential. The condition line is determined by successive approximations. An entire condition line is constructed for each approximation, and the resulting approximate line determines an auxiliary line used for the next construction. The method was used to calculate enthalpy transfer coefficients from experimental data on enthalpy transfer in a grid-packed tower for the tetrachloroethylene-air system, which are reported. Three types of countercurrent operations were studied: 1. The vapour content of the gas is increased and the gas is heated by hot liquid. 2. The vapour content of the gas is increased and both the gas and the liquid are cooled. 3. The liquid is heated by the hot gas and the vapour content of the gas is either increased or decreased. The enthalpy transfer coefficients for operations of the first type were lower than those for the other two. The existence of fog during dehumidification had no apparent effect on the coefficients. The application of the graphical method to the design of a cooler-condenser is described.