Ein Beitrag zur Verbesserung der Kammerfüllung von Schraubenmotoren
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Date
2003-11-10
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Universität Dortmund
Abstract
Derzeitige Schraubenmotorkonstruktionen basieren auf Entwicklungen aus dem Kompressorenbereich und weisen dementsprechend ein noch hohes Optimierungs-potenzial insbesondere der Energiewandlung auf. Untersuchungen zur Energiewandlung in Schraubenmotoren haben gerade für den Dampfschraubenmotor (aber auch für die übrigen Motorkonzepte) als wesentliche Verlustquellen Spaltverluste während der Kammerfüllung sowie Drosselverluste am Einlass ergeben. Primäre Aufgabe der Weiterentwicklung von Schraubenmotoren mit dem Ziel einer Steigerung der Energiewandlungsgüte muss es daher sein, die Füllung der Arbeitskammer zu verbessern.In der vorliegenden Arbeit werden die Ergebnisse aus den Untersuchungen zur geometrischen Optimierung des Füllvorgangs von Schraubenmotoren behandelt. Zu diesem Zweck werden Kennzahlen entwickelt, die eine rein geometrische Beurteilung unterschiedlicher Rotorgeometrien hinsichtlich dieser beiden Verlustanteile (Spaltverluste und Einlassdrosselung) ermöglichen. Untersuchungen von Rotor-geometrien anhand dieser Kennzahlen zeigen, dass eine einfache geometrische Variation von Rotorgeometrien, die heute im Bereich der Schraubenkompressoren üblich sind, für den Einsatz im Motorbetrieb nur ein geringes Optimierungspotenzial besitzen. Ursache sind die komplexen Zusammenhänge zwischen den Verlustmechanismen mit ihren häufig gegenläufigen Wirkungen auf die Energiewandlung.Verbessert wird diese Situation durch geometrische Veränderungen des Einlassbereiches in Form von Steuerscheiben auf den Rotoren sowie konischen Rotoren. Die dargestellten Zusammenhänge bleiben zwar auch für diese Geometrien erhalten, aufgrund der vergrößerten Einlassflächen ergibt sich jedoch eine, für den Motorbetrieb vorteilhafte, Verschiebung des Optimierungspotenzials hin zu größeren inneren Volumen-verhältnissen. Eine Reduzierung des Hauptrotorumschlingungswinkels kann hierbei ebenfalls zu einer Verbesserung der Energiewandlungsgüte durch verminderte Spaltverluste beitragen.Einen ersten Schritt zu einer Verbesserung der Einlassbedingungen in Schrauben-motoren stellt daher der Einsatz von profilierten Einlassscheiben auf Haupt- und Nebenrotor dar. Diese lassen sich unter eher geringem Aufwand in heutige Schraubenmotor-varianten integrieren. Ein deutlich höheres Potenzial zur Verbesserung der Energiewandlung bietet jedoch der Einsatz von konischen Rotoren.
This report deals with the quantification of two processes which take place during chamber filling and which are manly important for energy conversion in screw-type engines. Based on the experience of former investigations on screw-type engines it is known that on the one hand gas leakage through the clearances during the filling process and on the other hand pressure loss at the inlet site of the engine are of wide influence on energy conversion in screw-type engines. In this report a method for estimation of the chamber filling of screw-type engines and therefore of energy conversion will be presented.In a first step key values for analysing the two processes, gas leakage through the clearances during the filling process and pressure loss at the inlet, are shown. These key values are built by means of geometrical rotor parameters.In the following, geometrical variations of the rotor geometries are estimated via the key values (presented). (The variations of male rotor wrap angle, length / diameter ratio, number of lobes on male and female rotor and the inner volume ratio are calculated for an asymmetrical SRM-B-Profile).Especially for a large inner volume ratio, which is interesting for running the machine as an expander in a steam cycle, no optimal geometrical conditions can be found. A geometrical reduction of the operative clearance width, which should reduce the gas leakage too, always causes increased pressure loss at the inlet. Reducing the pressure loss causes increased gas leakage. But the key values for estimation of pressure loss are showing higher gradients for the examined geometrical variations. Therefore for a geometrical optimisation pressure loss at the inlet shut be considered first.In a next step three options for modification of the inlet area with regard to an improvement of the chamber filling are investigated. These are an inlet disc at one rotor (in the shown example at the male rotor), the use of conical rotors and finally inlet discs at the male and the female rotor.Based on the calculations for all variants a positive effect on chamber filling may be expected, but the option with an inlet disc at the male rotor shows the highest potential. It is mainly influenced by a reduction of gas leakage while the inlet situation shows no significant differences. But the volume of the inlet area at the disc operates like a kind of exhaust chamber which causes a loss of energy conversion. For an improvement of energy conversion by the advantage of an increased chamber filling it is necessary to design the volume of the exhaust chamber as small as possible.Conical rotors have shown also an increase of chamber filing. In contrast to the inlet disc it is mainly influenced by a reduction of pressure loss at the inlet. Therefore we can also expect an improvement of energy conversion by the use of conical rotors. But a final point to make is that manufacturing of these conical rotors is presently not investigated.The option of inlet discs at the male an the female rotor is an intermediate step between the cylindrical rotors and the conical ones. In this case the advantage of an increased chamber filling is combined with an easy way of manufacturing but without the disadvantage of an exhaust chamber. However the potential of improvement of chamber filling is less in comparison to the conical rotors.
This report deals with the quantification of two processes which take place during chamber filling and which are manly important for energy conversion in screw-type engines. Based on the experience of former investigations on screw-type engines it is known that on the one hand gas leakage through the clearances during the filling process and on the other hand pressure loss at the inlet site of the engine are of wide influence on energy conversion in screw-type engines. In this report a method for estimation of the chamber filling of screw-type engines and therefore of energy conversion will be presented.In a first step key values for analysing the two processes, gas leakage through the clearances during the filling process and pressure loss at the inlet, are shown. These key values are built by means of geometrical rotor parameters.In the following, geometrical variations of the rotor geometries are estimated via the key values (presented). (The variations of male rotor wrap angle, length / diameter ratio, number of lobes on male and female rotor and the inner volume ratio are calculated for an asymmetrical SRM-B-Profile).Especially for a large inner volume ratio, which is interesting for running the machine as an expander in a steam cycle, no optimal geometrical conditions can be found. A geometrical reduction of the operative clearance width, which should reduce the gas leakage too, always causes increased pressure loss at the inlet. Reducing the pressure loss causes increased gas leakage. But the key values for estimation of pressure loss are showing higher gradients for the examined geometrical variations. Therefore for a geometrical optimisation pressure loss at the inlet shut be considered first.In a next step three options for modification of the inlet area with regard to an improvement of the chamber filling are investigated. These are an inlet disc at one rotor (in the shown example at the male rotor), the use of conical rotors and finally inlet discs at the male and the female rotor.Based on the calculations for all variants a positive effect on chamber filling may be expected, but the option with an inlet disc at the male rotor shows the highest potential. It is mainly influenced by a reduction of gas leakage while the inlet situation shows no significant differences. But the volume of the inlet area at the disc operates like a kind of exhaust chamber which causes a loss of energy conversion. For an improvement of energy conversion by the advantage of an increased chamber filling it is necessary to design the volume of the exhaust chamber as small as possible.Conical rotors have shown also an increase of chamber filing. In contrast to the inlet disc it is mainly influenced by a reduction of pressure loss at the inlet. Therefore we can also expect an improvement of energy conversion by the use of conical rotors. But a final point to make is that manufacturing of these conical rotors is presently not investigated.The option of inlet discs at the male an the female rotor is an intermediate step between the cylindrical rotors and the conical ones. In this case the advantage of an increased chamber filling is combined with an easy way of manufacturing but without the disadvantage of an exhaust chamber. However the potential of improvement of chamber filling is less in comparison to the conical rotors.
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Keywords
Schraubenmotor, Kammerfüllung, Einlassdrosselung, Spaltverluste, Inneres Volumenverhältnis, Einlassscheibe, Konischer Rotor, screw-type engine, chamber filling, pressure loss, gas leakage, clearances, inner volume ratio, inlet disc, conical rotor