Martin Heimel

Fluid Flow in the Oil Pumping System of a Hermetic Compressor

This work deals with the investigation of the oil pumping system in hermetic compressors for refrigeration application. The oil pump which is used for this study consists of two pumping areas: the lower pumping area with a pick-up tube and an eccentric bore, and the upper pumping area with a helical groove. This study focuses on the helical groove in the upper pumping area. To analyse the fluid flow in the helical groove, a numerical approach is introduced. In this approach the Navier-Stokes equations are adapted to the problem and are solved by using the finite volume method. Compared to analytical models, this method is able to obtain the flow field in the cross section of the helical groove at higher resolution. The higher geometrical resolution also enables the analysis of the flow in the small gap between the rotating crankshaft and the stationary wall. The present method is used to quantify different operating parameters on the oil mass flow rate.

Transient 1D heat exchanger model for the simulation of domestic cooling cycles working with R600a

Generally, domestic refrigerators and freezers are running in non-continuous operation mode most of the time, which is a necessity to match cooling capacity to thermal loads. In currently available domestic appliances, this matching is realized either by on/off or variable frequency control of the hermetic compressor, leading to a repetitive and transient change of the system state. In case of longer compressor runtimes when cooling capacity demand is high (e.g., pull-down cycles, initial operation), steady-state operating conditions might be reached. The cycling transients cause losses in system efficiency; thus, they should be reduced or avoided. To understand the complex transient physical processes and to optimize the cooling system efficiency, the use of numerical methods has turned out to be a promising approach. For this reason, a 1D heat exchanger model, which has been successfully implemented in a domestic cooling cycle simulation tool, is presented in this work. The heat exchanger model is a further development of the model being presented in Berger et al. (2012). The same mathematical framework is used for modeling the evaporator and condenser. To compute the void fraction, pressure drop and heat transfer in the case of evaporation and condensation special empirical models, which are proposed in the literature, have been implemented. Finally, the numerical predictions are compared to experimental data gained from a purpose-built test rig.

Thermodynamic assessment of an innovative suction muffler for hermetic reciprocating compressors

The development of hermetic piston compressors in the past years has been driven by increasing the thermodynamic compressor performance. To identify loss mechanisms, existing systems have been analysed thoroughly by means of complex experiments and continuously improved numerical methods. The main loss mechanisms of a hermetic reciprocating compressor can be divided into electrical, mechanical and thermodynamic losses. Among the thermodynamic losses, the superheating of the working fluid in the suction line has a significant effect. Basic thermodynamic considerations show, that a gas temperature increase by 3K decreases the coefficient of performance (COP) by about 1% (Almbauer et al. (12)). In this work an innovative suction muffler concept is presented. Starting with a preliminary suction muffler concept, numerical simulations have been carried out to optimise the thermodynamic behaviour. Based on the simulation results, a first mock-up has been designed and implemented in an existing compressor system. Finally, the thermodynamic characteristics of the conventional and modified system are compared.

Industrial reed valve development using a virtual prototyping approach

ABSTRACT The gas dynamics of reciprocating compressors are significantly influenced by the dynamic behaviour of the reed valves. Basically, reed valves can be analyzed by experiments or numerical simulations. Experimental investigations of reed valve dynamics are extremely difficult: First of all, the compressors are of small size and therefore the valves are hardly accessible. Secondly, the application of measuring equipment tends to disturb the operating behaviour. For this reason, various numerical methods have been developed to capture the valve dynamics. Many methods are quite complex, so they cannot be used for industrial applications. In this work, two different generations of suction valves are compared and a virtual prototyping motivated suction valve development process is described. Various numerical simulations, beginning with simple steady state investigations up to a complex F luid S tructure I nteraction (FSI) approach, have been done to study the occurring physical effects in detail. Finally, the simulation results are discussed and the thermodynamic improvements of the new valve design are shown.

Measurement of Local Density Fluctuations for Combustion Diagnostics of Different Flames using Dual Laser Vibrometry

Experimental measurements of spatially- and frequency-resolved density fluctuations in two different types of flames, a non-premixed methane-jet flame and a premixed swirlstabilized methane-air flame were performed in this work. A novel interferometric measurement technique was applied for this purpose. Two laser vibrometers together with a signal analyser had been used to obtain frequency spectra of density fluctuations across both types of flames. Since laser vibrometry is based on interferometric techniques, the derived signals are path integrals along the measurement beam. To receive local frequency spectra of density fluctuations, long-time averaged time signals from each of the two systems were evaluated using correlation functions and cross spectra. Results obtained from measurements in the non-premixed jet flame and the premixed swirl-stabilized flame are compared and the application of the measurement technique for combustion diagnostics and detection and analysis of combustion instabilities is discussed.