José M. Corberán

Semiexplicit method for wall temperature linked equations (SEWTLE) : A general finite-volume technique for the calculation of complex heat exchangers

An in-depth analysis has been carried out on the discretization of a heat exchanger, the discretization of the governing equations, and the solution strategy for the resulting system of equations. An iterative global method to perform analysis of heat exchangers is proposed, called SEWTLE. This method provides flexibility to any flow arrangement or geometry, consideration of multiple streams, local evaluation of properties, friction factor, and heat transfer coefficient, and is characterized by good accuracy, high robustness, and fast computation time. In addition, three different numerical schemes for the discretization of the fluid and wall conservation equations at the cell level have been studied, and their advantages and disadvantages fully discussed.An in-depth analysis has been carried out on the discretization of a heat exchanger, the discretization of the governing equations, and the solution strategy for the resulting system of equations. An iterative global method to perform analysis of heat exchangers is proposed, called SEWTLE. This method provides flexibility to any flow arrangement or geometry, consideration of multiple streams, local evaluation of properties, friction factor, and heat transfer coefficient, and is characterized by good accuracy, high robustness, and fast computation time. In addition, three different numerical schemes for the discretization of the fluid and wall conservation equations at the cell level have been studied, and their advantages and disadvantages fully discussed.

Two-phase heat transfer analysis of evaporators

Abstract Customary evaporator or condenser characterisation provides data concerning the circulating mass flow rate and inlet and outlet conditions for each fluid flow. This paper analyses the estimation, from those typical experimental results, of the average heat transfer coefficient (HTC) taking place during the phase change region and its corresponding uncertainty. This analysis shows the influence of the typical measurement uncertainties of different measured magnitudes (temperatures, pressures and mass flow rates) and of operating conditions on the total uncertainty in the estimation of the two-phase HTC (2PhHTC). Additionally, the influence of the choice of heat exchanged, by the refrigerant or by the water, on the estimation of the 2PhHTC is also considered. The study is applied to a brazed plate heat exchanger (BPHE) working as an evaporator at both basic flow arrangements: co-current and counter-current. The results clearly show that the operating conditions in which this kind of equipment is typically characterised (and used) makes the estimation of the 2PhHTC difficult. The influence of the model used for the data reduction is also discussed. Finally, a sample of values of the 2PhHTC obtained from the analysis of the characterisation campaign results of a BPHE evaporator working with R22 is presented and briefly discussed.

Modelling Of Compact Evaporators AndCondensers

In this paper a model for compact evaporators and condensers is presented. The flow along a channel of a compact HE is considered to be 2-D and split into separated 1-D paths. Then, every 1-D flow path is discretised in as many elements as required. First, both fluid flow evolution along the heat exchanger are calculated through the integration of the 1-D conservation equations for single or two phase flow, assuming that wall temperatures are known. This calculation is performed through an explicit finite volume scheme, following the flow path. Once the temperature field for both fluids is calculated, then, the wall temperatures are obtained from the integration of the wall energy equation by means of an explicit scheme again. Then a new iteration starts till convergence throughout the HE is obtained. The presented method has proven to be very robust and very fast, being able to take into account local variation of properties and coefficients, and also include the calculation of longitudinal conduction.

Design of Efficient Air-Conditioning Systems for Electric Vehicles

This work has been supported by the European Commission under the 7th European Community framework program as part of the ICE project “MagnetoCaloric Refrigeration for Efficient Electric Air-Conditioning”, Grant Agreement no. 265434.B. Torregrosa-Jaime acknowledges the Spanish Science and Innovation Ministry (Ministerio de Ciencia e Innovacion) for receiving the Research Fellowship FPU ref. AP2010-2160.

Validation of a physiological model for controlling a thermal head simulator

The head plays an important role in human thermoregulation. Helmets typically provide additional thermal insulation that impairs heat dissipation, reducing comfort and user acceptance [1]. Thermal head manikins allow analysis of the local heat transfer properties of headgear, but they usually do not provide information about human thermal response. Physiological models allow simulation of local physiological reactions and the thermal effect at the skin surface. However, they cannot account for complex heat and mass exchange processes at the skin surface when protective equipment is worn. We aim at controlling a thermal head manikin with a physiological model to develop a novel advanced method for headgear evaluation. This work presents the validation of the aforementioned physiological model by Fiala [2,4] (FPC model version 5.3, Ergonsim, Germany) for prediction of global and local temperatures at the head-site, specially needed for the coupling with body part manikins, and is going to be used as a reference for validation of the coupled thermal head simulator.

Borehole modelling: a comparison between a steady-state model and a novel dynamic model in a real ON/OFF GSHP operation

The correct design and optimization of complex energy systems requires the ability to reproduce the dynamic thermal behavior of each system component. In ground source heat pump (GSHP) systems, modelling the borehole heat exchangers (BHE) dynamic response is especially relevant in the development of control strategies for energy optimization purposes. Over the last years, several models have been developed but most of them are based on steady- state approaches, which makes them unsuitable for short-term simulation purposes. In fact, in order to accurately predict the evolution of the fluid temperatures due to the ON/OFF cycles of the heat pump, it is essential to correctly characterize the dynamic response of BHE for very short time periods. The aim of the present paper is to compare the performance of an analytical steady-state model, available in TRNSYS environment (Type 557), with a novel short-term dynamic model. The new dynamic model is based on the thermal-network approach coupled with a vertical discretization of the borehole which takes into account both the advection due to the fluid circulating along the U-tube, and the heat transfer in the borehole and in the ground. These two approaches were compared against experimental data collected from a real GSHP system installed at the Universitat Politecnica de Valencia. The analysis was performed comparing the outlet temperature profiles predicted by both models during daily standard ON/OFF operating conditions, both in heating and cooling mode, and the between both approaches were highlighted. Finally, the obtained results have been discussed focusing on the potential impact that the differences found in the prediction of the temperature evolution could have in design and optimization of GSHP systems.

Experimental study of a heat pump with high subcooling in the condenser for sanitary hot water production

The use of heat pumps in order to produce sanitary hot water have been demonstrated as a very efficient alternative to traditional boilers. Nevertheless, the high water temperature lift (usually from 10°C to 60°C) involved in this application has conditioned the type of used solutions. In order to overcome it, transcritical cycles have been considered as the most suitable solution. The current article analyzes a new heat pump prototype able to enhance the heat pump efficiency using a subcritical cycle. The proposed prototype is able to control the system subcooling and make it capable to work at different subcoolings in the condenser. That kind of mechanism has demonstrated its capability to increase the efficiency of the heat pump. The obtained results have shown that coefficient of performance depends strongly on subcooling. In nominal condition (inlet/outlet water temperature at evaporator is 20°C/15°C and the water inlet/outlet temperature in the heat sink is 10°C and 60°C), the optimal subcooling is 42 K with a heating coefficient of performance of 5.35, which is about 25% higher than the same cycle working without subcooling.

Analysis of the Condensate Carryover Phenomenon on Fin and Tube Evaporators

Possible carryover of the condensate from the surface of the evaporator has always been a problem that in practice has been solved by limiting the air velocity. However, the need for more compact solutions and especially for the reduction of the frontal area in many applications requires the increase of the air velocity and therefore new solutions to overcome this problem must be developed. In this contribution, the authors develop an analytical model to estimate the evolution of the condensing drops over the fin surface of a heat exchanger as a function of the fin surface properties and air velocity. This model allows the estimation of the drop size when it starts to move and its trajectory and evolution along the fin. The possibility of drops forming water bridges in between the fins is also analyzed with estimation of the minimum fin separation to avoid its formation depending on the air velocity and the wettability of the fin surface. Finally, the results of an experimental campaign performed with two fin and tube coils of exactly same dimensions and geometry but with different fin materials: one with the standard aluminum fin and the other one with a specially outer hydrophilic layer, are presented, showing that this kind of coating avoids the condensate carryover with no appreciable penalty on the heat transfer performance.

Application of Magnetocaloric Heat Pumps in Mobile Air-Conditioning

Air-conditioning (AC) is an important sub-system in electric vehicles (EVs). AC is responsible for the highest energy consumption among all the auxiliary systems. As the energy is delivered by the batteries, the power consumption for air-conditioning can imply a significant reduction of the vehicle autonomy. Given the actual state of the art and the temperature and power requirements, electrically driven compressors are the most feasible solution. However, vapour-compression systems are reaching their maximum efficiency. Using innovative technologies can improve the performance of standard systems and hereby increase the vehicle autonomy. This paper presents the first steps in the design of a magnetocaloric air-conditioner for an electric minibus. The system will include two reversible magnetocaloric heat pumps, one in the front part of a minibus and one on the rear. The heat rejection system of the power electronics will be coupled to the air-conditioning system. In order to assist the design of the system, a dynamic model has been developed for the cabin, the hydraulic loops and heat exchangers, and the magnetocaloric units. An integrated design of the complete system is necessary, as it will work under dynamic conditions which depend on the thermal load in the cabin. In this paper, the operation conditions of the magnetocaloric units are presented and the design of the magnetocaloric air-conditioner is discussed. This work has been developed under the frame of the European Project ICE which aims to develop an innovative mobile air-conditioning system for EVs based on a magnetocaloric heat pump.

Interaction between natural convection and condensation heat transfer in the passive containment cooling condensers of the ESBWR reactor

Abstract The passive containment cooling condensers (PCCC) are the crucial part of several new reactor designs. In this paper we have developed several models to compute the heat transfer coefficients for the following cases: (i) condensation in presence of noncondensable gases inside the PCCC tubes; (ii) laminar natural convection for vertical cylinders; (iii) turbulent natural convection for vertical cylinders. These models have been implemented in the TRAC-BF1 code, and we have studied the interaction between the conservation regime inside the condenser tubes and the natural convection outside.