Phase change behavior of ammonia-water mixtures in corrugated plate heat exchangers

The present report describes the work realized in the frame of a Mas- ter Pro ject in collaboration between the company EnBW Energie Baden- Württemberg AG (Germany) and the Section of Mechanical Engineering of the Swiss Institute of Technology of Lausanne (EPFL) in view to obtain 30 ECTS credits and the title of Master of Science in Mechanical Engineering. The general goal is to reach a better understanding of the heat exchange processes occuring in corrugated plate heat exchangers. EnBW AG operates a geothermal power plant based on the Kalina cycle, applying corrugated plate heat exchangers. As working fluid of the power cycle a zeotropic mixture formed by ammonia and water is used. Heat source of the geothermal power plant in Bruchsal (D) is a natural hydrothermal system. Zeotropic mixtures present specific properties complicating their use in heat exchangers. The mix- ture components' different boiling points create a temperature glide during the mixture's phase change, in contrary to pure fluids that vaporize at a constant temperature. In addition, a composition difference appears between the liquid phase and the vapor phase, which induces a diffusion mass transfer resistance. The several studies conducted on the topic show that both effects combined reduce the heat transfer performances in phase change. The modelling of the mixture properties and behavior is realized in an in- dependent manner by using correlations from the literature. Specific enthalpy and entropy are notably modelled with the correlations by Ziegler and Trepp. The heat transfer coefficients are computed using correlations by Dovíc et al. for the monophasic flows and by Han et al. for biphasic flows. The calcula- tion of pressure drops is discussed and correlations by the same authors are presented. Unfortunately the pressure drop calculation could not be imple- mented in the time allowed to this work. However their implementation does not present special technical difficulties. The results of the calculation for realistic conditions show good agreement with the available experimental data. More data are however needed to obtain a complete validation of the model. A sensitivity analysis is performed to determine the effect of the change of certain operating conditions or of the heat exchanger design. The influence of the following parameters is studied: number of plates, length of those, geothermal brine inlet temperature and mass flow and ammonia concentration of the mixture. It appears that the most influent parameter on the recovered heat load is the inlet brine temperature. Adding plates to the heat exchanger only permits a limited gain of performances. Lengthening the plates is pre- ferred. An augmentation of the mass velocity of the brine, which seems to be the limiting factor in the heat transfer coefficients values, seems also not to permit a significant performance increase. The sensitivity analysis shows though that it is possible to react to a brine temperature change by modifying the ammonia concentration of the mixture. It is therefore possible to adapt the cycle to the conditions, as one wants to maintain a constant turbine inlet temperature or to reach better exergy efficiency. This result must however be reconsidered through a wider perspective including the entire thermodynamic cycle. Given the results listed here above, one can deduce that the relatively low temperature of the geothermal brine strongly limits the possibilities of 3 optimization. The adapted ammonia concentration range is narrow. In certain cases the ob jective of maintaining the amount of transferred thermal power and the one of keeping good exergy efficiency are conflicting. It could however be instructive to lead a sensitivity analysis on the cycle pressure level. This analysis would though have to be included in a global view of the cycle given the numerous consequences. Finally, the results produced by the model allow to clearly illustrating the advantage of zeotropic mixtures regarding the exergy efficiency of the heat exchange. The efficiency is very high, reaching 90% in certain conditions, which is not realizable in the same conditions using a pure working fluid. The study of the behavior of zeotropic fluids should be pursued in near future in the view of a more rational use of the energy.

Efficient air-water heat pumps for high temperature lift residential heating, including oil migration aspects

Michele Zehnder

This thesis presents a system approach with the aim to develop improved concepts for small capacity, high temperature lift air-water heat pumps. These are intended to replace fuel fired heating systems in the residential sector, which leads to a major reduction of the local greenhouse gas emissions. Unfavorable temperature conditions set by the existing heat distribution systems and by the use of atmospheric air, as the only accessible heat source, have to be overcome. The proposed concepts are intended to cover the total application range and to provide the full heat demand without any additional (electric) heat supply. A systematic approach, using a multi objective optimization tool, has been applied to evaluate possible alternate refrigerants, which perform best, regarding to the system COP and the specific heat output. All the optimal refrigerant blends are composed by flammable refrigerants and a potential increase of the COP of 8% (compared to the commercial blend R-407C) has been determined. These potential improvements highly depend on the acceptance to use flammable refrigerants, as can be shown by this evaluation. The standard concepts of small capacity heat pumps suffer from a restricted application range and show highly decreasing performances (COP and provided heat) at extreme operating conditions. From the examination of the thermodynamic cycle, different improved concepts are proposed and are assembled in a generalized super-configuration. The most promising concepts have been built as prototype units and have been tested in laboratory. These concepts are: a) the two-stage compression cycles with economizer heat exchanger or b) with economizer flash tank at the intermediate pressure level, c) the booster-compressor setup, d) the one-stage compression cycle including a new developed hermetic compressor, which enables intermediate injection of saturated vapor flow and e) a small capacity auxiliary cycle for liquid subcooling. In all these concepts the application range could be extended, by achieving reduced discharge temperatures and the heat rate provided at extreme operating conditions could be substantially increased (20%-35%, +100% for the booster setup), using the same compressor and evaporator size. System COP has been improved (~5%) over a large application range, and specifically in high temperature lift operating conditions. The experimental evaluation reveals the major problem of unbalanced oil migration in two-stage compression cycles (except for the booster concept). An extensive evaluation has been applied, to analyze the oil migration in two-stage compression heat pumps. A new developed measurement technique, using a Fourier Transform Infrared Spectrometry combined with a high pressure supporting ATR cell, has been calibrated (with a lower detection limit at 0.2% - 0.4%, and a sensitivity of 0.1%) with the used refrigerant-oil mixtures (R-134a/POE oil and R-407C/POE oil). It has been applied to perform on-line oil concentration measurements during two-stage and one-stage steady stage and during transitory operating modes. A generalized steady state simulation model has been developed including namely the new developed compressor model with an intermediate injection port, considering the geometrical flow path of the tested prototype compressors. A flow map based extensive heat transfer model is integrated into a finned tube evaporator model and taking into account oil effects on the heat transfer. General models of plate heat exchangers and for capillary tube expansion devices complete this modular simulation model, on which the concepts of the super-configuration can be calculated and some parametric analysis has been performed. An in house developed fluid interface module includes the fluid properties calculation program Refprop and allows to define new mixtures or to use the large number of predefined refrigerants.

EPFL2004

Two-Phase Heat Transfer Mechanisms within Plate Heat Exchangers

Raffaele Luca Amalfi

Plate heat exchangers are adopted in many domestic and industrial applications, such as ventilation, air conditioning, evaporation or condensation process, heat pumps and cooling of hydrodynamic circuits in engines. In the present thesis, an extensive experimental study to characterize thermal and hydraulic performance of a compact plate heat exchanger has been carried out and then follow up with proposition of new prediction methods for single-phase and evaporating flows in the plate¿s channels, in the form of general correlations that include large additional databases culled from the literature. Specifically, upward single-phase flow and flow boiling heat transfer of low pressure liquid refrigerants were investigated within a plate heat exchanger prototype fabricated with 1 mm pressing depth and chevron angle of 65°. High spatial and temporal resolution infrared measurements were implemented to obtain local (pixel-by-pixel) heat transfer coefficients and frictional pressure drops. Single-phase experiments were carried out for Reynolds numbers ranging from 34 to 1615 and Prandtl numbers from 4.9 until 6.5, and the associated effect of the main involved parameters on the thermal and hydraulic performance was analyzed in detail. Several of the most quoted prediction methods available in the literature were statistically evaluated against the present heat transfer and pressure drop database and a new models were proposed to predict mean and local thermal and hydraulic performance of the present compact plate heat exchanger. Two-phase experiments were carried out for mass fluxes ranging from 10 kgm-2s-1 to 85 kgm-2s-1, imposed heat fluxes from 225 Wm-2 to 4100 Wm-2, saturation temperatures from 19°C to 35°C and vapor qualities from 0.05 to 0.90. The heat transfer coefficient increased with mass flux, heat flux and saturation temperature (system pressure) whilst the frictional pressure drop rose with mass flux and vapor quality but decreased with saturation temperature. Based on local infrared measurements, a new correlation for predicting local frictional pressure gradient through the test section was proposed. Next, a wide experimental databank was culled from thirteen literature research studies, which investigated flow boiling heat transfer and two-phase frictional pressure drop within chevron plate heat exchangers. The database was first adopted to validate the predicted capabilities of 29 literature models, and then utilized to provide the new prediction methods to evaluate local heat transfer coefficients and frictional pressure gradient. These new models were developed from 1903 heat transfer and 1513 frictional pressure drop data points (3416 total) and were proved to work better over a very wide range of operating conditions, plate designs and fluids (including ammonia). The general flow boiling prediction methods have been programmed into a simulation code to analyze local thermal and hydraulic performance of the plate heat exchangers over a large range of test conditions. The simulation results were then validated against independent experimental database from literature, resulting in very good agreement between each other. The present simulation code represents a powerful tool to be used for practical applications by the thermal engineers in order to design and rate commercial plate heat exchangers.

EPFL2016

Flow boiling and frictional pressure gradients in plate heat exchangers. Part 2: Comparison of literature methods to database and new prediction methods

Raffaele Luca Amalfi, John Richard Thome, Farzad Vakili Farahani

In the second part of this study a sensitivity analysis on the prediction methods is performed to consider the effect of plate geometry on thermal hydraulic performance and an extensive comparison of all the two-phase pressure drop and flow boiling heat transfer prediction methods available in the open literature are also provided versus the large diversified database presented in Part 1. The experimental databank, from numerous independent research studies, is then utilized to develop the new prediction methods to evaluate local heat transfer coefficients and pressure drops. These new methods were developed from 1903 heat transfer and 1513 frictional pressure drop data points (3416 total), respectively, and were proved to work better over a very wide range of operating conditions, plate designs and fluids (including ammonia). The prediction for flow boiling heat transfer coefficients was broken down into separate macro- and micro-scale methods. (C) 2015 Elsevier Ltd and International Institute of Refrigeration. All rights reserved.