Abstract: Ammonia is an environment friendly natural refrigerant with attractive thermo-physical properties. Experimental heat transfer and pressure drop data are obtained for evaporation of liquid ammonia in a commercial plate heat exchanger for symmetric 60°/60° (hard) chevron angle plates. Experiments were carried out for mass flux ranging from 8.5 to 27 kg m−2 s−1 at saturation temperatures ranging from −25 °C to −2 °C. The heat flux was varied between 21 kW m−2 and 44 kW m−2. The heat transfer coefficient increased with an increase in saturation temperature and mass flux. Furthermore, heat transfer coefficient was observed to increase with exit vapor quality. The friction factor decreased with exit vapor quality and equivalent Reynolds number, while it increased with the fluid temperature. The work reveals that ammonia has far better heat transfer and pressure drop characteristics compared to HFCs. Two phase Nusselt number and friction factor correlations are also proposed.

Abstract: Multistream plate fin heat exchangers have replaced two-stream heat exchangers in diverse applications due to their compactness, capacity of handling multiple fluid streams in a single unit, and possibilities of having intermediate entry and exit of the streams. Unique features of such heat exchangers like direct/indirect crossover in temperatures due to several thermal communications among the fluid streams and the dependence of the thermal performance on “stacking pattern” have no equivalent in two-stream modules. As a consequence, an extension of the commonly used design/simulation techniques like ϵ-NTU or the LMTD method, applicable for two-stream exchangers, fails miserably in the case of multistream units. Though several techniques have been suggested over the years, in reality, no universally accepted methodology exists for the “thermal design” of multistream plate fin heat exchangers to date. In this communication, a state-of-the-art review of the thermal design of multistream plate fin heat excha...

Abstract: The horizontal-tube falling film evaporation is a widely adopted technique in multiple-effect distillation desalination plant. It has a high heat transfer coefficient under quite small temperature difference. In this paper, an experimental equipment for horizontal-tube falling film evaporation was set up. Experiments were carried out to show how the heat transfer coefficient is affected by different parameters including heat flux, circumference direction of tubes, spray density, evaporation temperature, and experimental fluid. Results indicate that the heat transfer coefficient decreases after a little increase with growth of spray density. The heat transfer coefficient decreases along the tube circumference, but at the bottom of the tube, it shows increasing trend. In addition, a simple comparison between seawater and fresh water in heat transfer coefficient is also provided.

Abstract: A simulation model of the mini-channel evaporator using R1234yf as working fluid is developed in the present study. The finite element conception is employed, and the effectiveness-NTU method is used to calculate the heat transfer rate. For the two-phase heat transfer of the refrigerant, six correlations are compared in the developed model. The calculated and experimental data of six different samples with R1234yf were compared under typical working conditions of an automotive air conditioning system. The developed model using Kandlikar's correlation to calculate for the two-phase heat transfer coefficient of R1234yf can obtain the highest precision. The distribution of the local two-phase heat transfer coefficients in the evaporator is studied using the developed model. Under the same conditions, R1234yf has a lower two-phase heat transfer coefficient than R134a. In high quality region, the heat transfer coefficients of both refrigerants decrease with the increasing of the refrigerant quality.

Abstract: The link between heat transfer intensity and hydraulic resistance of plate heat exchangers channels is determined with the use of modified Reynolds analogy of heat and momentum transfer. The formula to estimate the share in total hydraulic resistance of pressure loss due to friction is proposed. The resulting model enables to calculate film heat transfer coefficients in plate heat exchangers channels on a data of hydraulic resistance of the main corrugated heat transfer field. The calculations are compared with the experimental results on heat transfer in channels with cross corrugated walls available in the literature. The good agreement confirms the assumptions made on deriving the equation of the analogy between heat and momentum transfer in proposed form in plate heat exchanger channels.

Abstract: The thermal performance of a Z-shaped enthalpy heat exchanger utilizing 45-gsm Kraft paper as the heat and moisture transfer surface for heating, ventilation, and air conditioning (HVAC) energy recovery is experimentally investigated through temperature and moisture content measurements. A mathematical model is developed and validated against the experimental results using the effectiveness-NTU method. In this model the paper moisture transfer resistance is determined by paper moisture permeability measurements. Results showed that the paper moisture transfer resistance is not constant and varies with moisture gradient across the paper. Furthermore, the model is used to predict the heat exchanger performance for different heat exchanger flow configurations. The results showed that higher effectiveness values are achieved when the heat exchanger flow path width is reduced. Temperature and moisture distribution in the heat exchanger is also studied using a computational fluid dynamics package (FLUENT). To m...

Abstract: Heat transfer coefficients for enhanced tubes are typically measured indirectly using the “Wilson plot” method to first characterize the thermal performance of the one side (heating or cooling supply) and then to obtain the heat transfer data for the enhanced side based on the Wilson plot results. A brief history of the Wilson plot evolution and alternative methods to the Wilson plot, including the advantages and disadvantages, are discussed as applied for enhanced heat transfer. A slight modification to the Briggs and Young (1969) method is proposed so that the experimental errors can be propagated through the method, allowing us to estimate the error in the generated correlations. Furthermore, a new method based on unconstrained minimization is proposed as an alternative to the least-squares regression. As an example, both methods have been applied to two enhanced boiling tubes (the most recent generation) and heat transfer coefficients were compared against direct wall temperature based heat transfer c...

Abstract: Compact heat exchangers (CHEs) are characterized by a high surface area per unit volume, which can result in a higher efficiency than conventional heat exchangers. They are widely used in various applications in thermal fluid systems including automotive thermal fluid systems such as radiators for engine cooling systems. Recent development of nanotechnology brings out a new heat transfer coolant called “nanofluids,” which exhibit larger thermal properties than conventional coolants due to the presence of suspended nanosized composite particles in a base fluid. In this study, a numerical investigation using different types of nanoparticles in ethylene glycol-base fluid namely copper (Cu), diamond (DM), and silicon dioxide (SiO2) on automobile flat tube plate-fin cross-flow CHE is explored. The nanoparticles volume fraction of 2% is considered for all types of nanofluids examined in this study. The three-dimensional (3D) governing equations for both liquid flow and heat transfer are solved using a standard finite volume method (FVM) for the range of Reynolds number between 4000 and 7000. The standard 𝜅-𝜀 turbulence model with wall function is employed. The computational model is used to study the variations of shear stress, skin friction, and convective heat transfer coefficient. All parameters are found to yield higher magnitudes in the developing and developed regions along the flat tubes with the nanofluid flow than base fluid. The pressure drop is slightly larger for nanofluids but insignificant at outlet region of the tube. Hence, the usage of nanofluids in CHEs transfers more energy in a cost-effective manner than using conventional coolants.

Abstract: In this study, a tube-in-tube heat exchanger model applicable to supercritical CO2 and water was developed. The developed model is first validated with some existing measurements. Normally, the variation of the heat transfer rate for a constant-property working fluid shows a monotonic decrease from the inlet of minimum heat capacity flow rate (Cmin). By contrast, the CO2 may present a local minimum and a local maximum along the length of the heat exchanger, provided CO2 passes through the pseudo-critical temperature, and this phenomenon becomes more and more pronounced when the pressure is close to the critical pressure. In contrast, it is possible for a local maximum heat transfer rate to occur near the inlet of Cmin even when the CO2 does not pass through the pseudo-critical point. This happens when Cmin is on the water side and the property variation of CO2 is taken into account. The calculation also shows that the effect of the inlet pressure on the variation of the CO2 temperature is not as apparent as the effect of the inlet pressure on the heat transfer rate, even when there is a significant change in the overall heat transfer coefficient, implying that the heat transfer characteristics of CO2 near the pseudo-critical region is similar to normal refrigerants, which show an invariant temperature at the condensation point. Hence, it would be beneficial to extend the influence of the pseudo-critical region when taking the heat transfer augmentation into consideration.

Abstract: In this paper an analysis of laminar heat transfer and fluid flow in a wavy fin-and-tube heat exchanger has been carried out. Three-dimensional (3D) numerical simulation results of a circular tube heat exchanger were compared with published numerical and experimental results. The computational fluid dynamics (CFD) procedure was validated by comparing average Nusselt numbers, and good agreement between published and calculated results has been accomplished. The influence of inlet air velocity, varying from 0.5 to 5 m s−1, as well as fin pitch, varying from 0.4 to 4 mm, on heat transfer and pressure drop conditions has been studied. The results have shown that there is an optimal fin pitch for each air velocity, which gives the best heat exchanger performance from the heat transfer point of view.