Abstract The performance of a falling-film heat exchanger is strongly linked to the surface characteristics and the heat transfer processes that take place over the tubes. The primary aim of this numerical study is to characterize the influence of surface wettability of tubes on the falling film flow mode and its associated surface heat transfer. Surface wettability is generally characterized by the contact angle and, in this study, the wettability characteristics ranged from superhydrophilic to a superhydrophobic tube surface. The dynamic motion of the triple contact line connecting the solid, liquid and gas phases over the tube surface is replicated with the help of the Kistler’s dynamic contact angle model. The volume of Fluid (VOF) simulations was carried out for the flow and heat transfer of liquid flow over horizontal tubes with different surface wettabilities. The mass flow rate is such that the flow is in the jet mode where the liquid flows in the form of jets in between the horizontal tubes. This corresponds to a liquid mass flow rate per unit tube length of 0.06 and 0.18 Kg/m-s, under which the inline and staggered jets modes of flow are observed. Under the two flow rates and different surface wettabilities, the liquid flow hydrodynamics over the tube surfaces was explored in terms of the liquid film thickness, the contact areas (solid-liquid and liquid-air) between the different phases, and the heat transfer coefficient. The axial resistance imposed by the increasing contact angle tends to inhibit the extent of the liquid spreading over the tube surface and this, in turn, influences the liquid film thickness and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle ranged from 2° to 175°. Heat transfer characteristics were quantified over a wide range of contact angles for the two mass flow rates. Fluid recirculations were observed in the liquid bulk which had a major impact on the heat transfer distribution over the tube surface.

Heat exchangers using in-tube condensation have great significance in the refrigeration, automotive and process industries. Effective heat exchangers have been rapidly developed due to the demand for more compact systems, higher energy efficiency, lower material costs and other economic incentives. Enhanced surfaces, displaced enhancement devices, swirl-flow devices and surface tension devices improve the heat transfer coefficients in these heat exchangers. This study is a critical review on the determination of the condensation heat transfer coefficient of pure refrigerants flowing in vertical and horizontal tubes. The authors’ previous publications on this issue, including the experimental, theoretical and numerical analyses are summarized here. The lengths of the vertical and horizontal test sections varied between 0.5 m and 4 m countercurrent flow double-tube heat exchangers with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The measured data are compared to theoretical and numerical predictions based on the solution of the artificial intelligence methods and CFD analyses for the condensation process in the smooth and enhanced tubes. The theoretical solutions are related to the design of double tube heat exchangers in refrigeration, air conditioning and heat pump applications. Detailed information on the in-tube condensation studies of heat transfer coefficient in the literature is given. A genetic algorithm (GA), various artificial neural network models (ANN) such as multilayer perceptron (MLP), radial basis networks (RBFN), generalized regression neural network (GRNN), and adaptive neuro-fuzzy inference system (ANFIS), and various optimization techniques such as unconstrained nonlinear minimization algorithm-Nelder-Mead method (NM), non-linear least squares error method (NLS), and Ansys CFD program are used in the numerical solutions. It is shown that the convective heat transfer coefficient of laminar and turbulent condensing film flows can be predicted by means of theoretical and numerical analyses reasonably well if there is a sufficient amount of reliable experimental data. Regression analysis gave convincing correlations, and the most suitable coefficients of the proposed correlations are depicted as compatible with the large number of experimental data by means of the computational numerical methods.

Heat exchangers using in-tube condensation have great significance in the refrigeration, automotive and process industries. Effective heat exchangers have been rapidly developed due to the demand for more compact systems, higher energy efficiency, lower material costs and other economic incentives. Enhanced surfaces, displaced enhancement devices, swirl-flow devices and surface tension devices improve the heat transfer coefficients in these heat exchangers. This study is a critical review on the determination of the condensation heat transfer coefficient of pure refrigerants flowing in vertical and horizontal tubes. The authors’ previous publications on this issue, including the experimental, theoretical and numerical analyses are summarized here. The lengths of the vertical and horizontal test sections varied between 0.5 m and 4 m countercurrent flow double-tube heat exchangers with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The measured data are compared to theoretical and numerical predictions based on the solution of the artificial intelligence methods and CFD analyses for the condensation process in the smooth and enhanced tubes. The theoretical solutions are related to the design of double tube heat exchangers in refrigeration, air conditioning and heat pump applications. Detailed information on the in-tube condensation studies of heat transfer coefficient in the literature is given. A genetic algorithm (GA), various artificial neural network models (ANN) such as multilayer perceptron (MLP), radial basis networks (RBFN), generalized regression neural network (GRNN), and adaptive neuro-fuzzy inference system (ANFIS), and various optimization techniques such as unconstrained nonlinear minimization algorithm-Nelder-Mead method (NM), non-linear least squares error method (NLS), and Ansys CFD program are used in the numerical solutions. It is shown that the convective heat transfer coefficient of laminar and turbulent condensing film flows can be predicted by means of theoretical and numerical analyses reasonably well if there is a sufficient amount of reliable experimental data. Regression analysis gave convincing correlations, and the most suitable coefficients of the proposed correlations are depicted as compatible with the large number of experimental data by means of the computational numerical methods.

The manifestation of unsteady heat transfer in incompressible forced convecting flows, such as film flows, is of direct practical interest to a large number of important industrial applications, such as film evaporators and reactors. In this paper an attempt is made to improve our fundamental understanding of unsteady heat transfer in these flows, and to predict this phenomenon by developing a reduced model of the specific case of a film flow over a heated solid. A time-varying heat transfer fluctuation is imposed, caused by a feature or disturbance that convects freely with the flow. The energy equation is solved to obtain the fluid’s thermal development, while a conjugate boundary condition comprising a one-dimensional thermal interaction between the fluid and the contacting solid substrate is imposed. Two heat transfer behaviours are found, which are referred to as “under-damped” and “damped”. Their characteristics with regards to the thermal development of the investigated flows are studied, including a consideration of heat transfer “augmentation” (relative to equivalent steady flows). Augmentation is found to remain negative for all investigated conditions.

The surface evaporation of the extended meniscus liquid film in the micro channel driven by the capillary force is of the key process in a lot of heat sink and thermal management equipments. The physio-mathematical model was established considering the variation of the apparent contact angle to describe the configuration of the total extended thin film region, including that of equilibrium, interline and intrinsic meniscus regions. In additions, the minimum energy principle was employed to determine the range of thin film interline region. The results indicate that the range of thin film interline region is quite small in the total extended film region, nevertheless the variation of its surface temperature is much obvious, implying that the resulted Maragoni effect can cause great “pump” force to drive the film flow, which may be the main contribution of the interline region to the total extended thin film evaporation. The variations of evaporating rate, and also the resulted mean velocity of liquid film in the thin film interline region under different wall temperature and apparent contact angle were revealed, which showed that there existed the local maximum value of evaporating rate in this region by the coupled influences of the decreased film thickness and variation of the interface temperature.

Critical heat flux experiments of subcooled, thin, and high-velocity water flow were performed. The test flow channel was rectangular. The width of the flow channel was 2 mm and the height was 0.5 mm or 0.2 mm. The heat transfer surface was 2 mm × 2 mm. At the low heat flux, tinny bubbles were formed at the downstream part of the test heating surface. As the heat flux was increased, the bubble diameter increased and the coalescence of bubbles occurred. Then, the coalesced bubbles grew larger to cover the whole area of the heat transfer surface. Finally, the dried area appeared at the downstream end of the heat transfer surface to cause the critical heat flux condition. The critical heat flux was considerably higher than that of the subcooled flow boiling for the usual-size pipe as well as those of the saturated and the subcooled pool nucleate boiling. As the flow rate was increased, the period between the onset of boiling and the critical heat flux occurrence became narrow. The critical heat flux in the present experiments where the heat transfer surface was located at the just downstream of the flat channel outlet was considerably larger than those in the previous experiments where the heat transfer surface was located at the outlet end of the flat channel or the upstream of the outlet. By producing a fast jet along the surface and providing enough space for generated bubbles to leave from the surface, the critical heat flux was considerably augmented. Critical heat fluxes obtained in the present experiments were in in-between of the correlations for the flowing-upward film flow and for the flowing-downward film flow. The increasing trend for the flow rate was similar to that of the correlations.

Numerical simulations based on the MARS (Multi-interface Advection and Reconstruction Solver) developed by one of the authors have been performed in order to understand the heat transfer mechanism of falling film flows along a vertical wall. In the present study, we focused on an internal structure, i.e. velocity profile and heat transfer. In this study, the velocity profiles in a falling film are categorized into three regions: substrate film region, solitary wave region, and capillary wave region with each characteristic. Moreover, small vortices can be found in the capillary wave region. Additionally, numerical results indicate that heat transfer mechanism is different for these three regions. We proposed new heat transfer evaluation manner as a function of the Prandtl number for these regions. This manner could contribute to a design work of falling film flow device.