Juergen Koehler

A validated numerical-experimental design methodology for a movable supersonic ejector compressor for waste-heat recovery

The aim of this paper is to develop the technical knowledge, especially the optimum geometries, for the design and manufacturing of a supersonic gas-gas ejector for a waste heat driven vehicle cooling system. Although several studies have been performed to investigate the effects of geometrical configurations of gas-gas ejectors, a progressive design methodology of an ejector compressor for application to a vehicle cooling system has not yet been described. First, an analytical model for calculation of the ejector optimum geometry for a wide range of operating conditions is developed, using R134a as the working fluid with a rated cooling capacity of 2.5 kW. The maximum values of entrainment ratio ( ) have been estimated by correlation of the main parameters in a non-dimensional form. The optimum values of nozzle throat diameter ( ) and mixing chamber diameter ( ) thus obtained are used as a starting point for the CFD optimization covering a wide range of geometrical configurations. To assess the effect of various dimensional quantities, an optimization technique has been proposed for calculation of the most efficient geometry of the target ejector for manufacturing. Using a vehicle cooling system as a test case, the final optimized dimensions are reported and discussed. An experimental validation confirms the CFD results and the ejector performance with a normalized deviation of 5% between observed and simulated results, demonstrating that the methodology is a valid ejector design tool for a wide range of applications.

Visual Investigation of an Ejector Motive Nozzle

Previous investigations by other authors, e.g. Lorentzen [1], have shown that in a conventional refrigeration cycle significant throttling losses occurs. With the help of an ejector, these losses can be reduced. As a result, the energetic efficiency (COP) of the refrigerant system will be improved. Investigations show that CO2 ejector cycles are feasible and that some systems have already been commercialized successfully. The key issues in the optimization of the ejector used in a refrigeration cycle are the geometries of the different ejector parts. To optimize the geometry, a deeper understanding of the physical effects and the flow conditions within the ejector are essential. So far there are only a few investigations published on this issue, e.g. Elbel [2], investigated the flow in the mixing section of the ejector. This paper presents experimental results for different ejector nozzle geometries and operational condiditons. The motive nozzle was investigated separately from the other ejector parts. Investigated were multi-hole nozzles and the effect of the jet shape. Parameters were chosen according to the typical conditions in ejector refrigeration systems. Based on these conditions, the free jet exiting the motive nozzle was observed. To investigate the jet shape, an new experimental setup was designed. The motive jet was visually observed in a glass cylinder. The combination of both the contraction and compressibility effect on mass flow rate was also investigated.Copyright

Use of ab initio interaction energies for the prediction of phase equilibria in the system nitrogen–ethane

Ab initio molecular orbital methods have been used for calculations of interaction parameters of the UNIQUAC and NRTL activity coefficient model to predict phase equilibria in the system nitrogen–ethane. The UNIQUAC and NRTL model with ab initio parameters, calculated on the MP4 and QCISD(T) theory level, have been used in different gE-mixing rules to predict high-pressure vapor–liquid- (VLE) and vapor–liquid–liquid-equilibria (VLLE) by the Peng–Robinson and the Soave–Redlich–Kwong equation of state. The results have been compared to predictions based on UNIFAC with the PSRK-mixing rule. The results using ab initio-UNIQUAC are poor. However, ab initio-NRTL gives good VLE-predictions with all gE-mixing rules and with both equations of state. Only in the temperature range below 133 K, where the VLLE occur, is ab initio-NRTL ninferior to the predictions by UNIFAC.

Measurements of Local Heat Transfer Coefficients in Heat Exchangers With Inclined Flat Tubes by Means of the Ammonia Absorption Method

For high efficiency compact heat exchangers one needs to gain detailed knowledge of the distribution of the local heat transfer. For a profound assessment of heat enhancing mechanisms like secondary flow structures which are often found at rather small scales it is necessary to perform heat transfer measurements with high spatial resolution. A technique that satisfies this need is the ammonia absorption method (AAM). It is based on the analogy between heat and mass transfer. The here presented paper describes a new calibration approach for the AAM. It is done through the use of a well established heat transfer correlation for the hydrodynamic and thermal entry in parallel plate channels. This calibration approach is applied to heat transfer measurements in compact heat exchangers with inclined flat tubes and plane fins at Redh = 3000 . The heat transfer performance is compared to fin-and-tube heat exchangers with round tubes. It is found that the novel devices show consistently higher global Nusselt numbers than comparable round tube heat exchangers.Copyright