Assisting Nozzle Pressure Design

30 No 22 Autumn 2002 by Moustafa Abdel-Maksoud and Hans-Jürgen Heinke , SV A Potsdam, Germany Nozzle propellers are used in many types of ships and for dynamic positioning of platforms. Their operation at high thrust loadings and low inflow velocities requires high-tech designs. SV A Potsdam specializes in these propulsion systems and is investing in continuous research to ensure that its designs meet expectations. The flow in the gap between the propeller tip and the inside wall of the nozzle strongly influences the performance of the propeller , and the greater the thrust loadings, the greater the influence. CFD allo ws visualization of these flows, which are difficult to study experimentally because the gap can be less than one millimetre. Unfortunately , the required computational effort increases rapidly as the thrust loading increases (due to slower conv ergence as the rotational speed of the propeller increases), and we hav e realised that combining experimental and numerical investigation s c an overcome the shortcomings of each method. The experimental data p ro vide a good overview of the forces and moments for the whole range of operational conditions, while CFD re veals the detail of the flow for selected loadings. In our CFD model, we use the powerfu l rotor/stator capabilities of CFX-T ASCflow . The stationary p art includes the inflow and outflow regions and the nozzle; the propeller bl ade and part of the propeller shaft are included in the rotating part. The flow in and ar ound the nozzle varies strongly with Reynolds numbe r. As this incr eases,the press ure reduction near the leading edge is increased and the separation point near the trailing edge mov es backwards. This leads to a change in the operating point of the propeller. T o understand these phenomena fully , we undertook an extensive CFD analysis of the flow behaviour of the nozzle at different thrust loading conditions and Reynolds number, investigating also different nozzle and propeller geometries. The results compared well with both published data and LDV measurements behind the nozzle. T o analyse the effect of scale on the performance characteristic of our design, we compar ed the numerical results of three full-scale propellers with the corresponding coefficients of a model propeller. The results show ed that with increasing Reynolds n umber, the thrust coefficient of the nozzle increases (this effect is stronger for low thrust loadings), while the torque and thrust coefficients of the propeller decrease. The flow velocity through the nozzle at full scale is higher than at model scale. This increases the efficiency of the nozzle at full-scale and reduces the propeller loading (thrust and torque coefficients). Further reduction of the propeller torque takes place due to the lower friction coefficient on the propeller blades at higher Reyno lds numbers. This is one possible explanation for the strong Reynolds-number dependency of the torque coefficient of the ducted propeller in comparison to a free-running one. The quality of propeller design is directly affected by the ability to accurately predict thrust and torque loading at full scale. The results of the Assisting nozzle pr opeller design Influence of Reynolds number and the thrust-loading coefficient on propeller torque coefficients. The figure includes the calculated ratio of the torque coefficients (KQs) as a function of t he corresponding coeffici ents of a model propeller. The calculated forces and torque acting on the propeller are in good agreement with published results, showing the propeller thrust coefficient (KTP), torque coefficient (KQ), nozzle thrust coefficient (KTD) and total thrust coefficient (KTG). The detailed results of the study have been published in the 24th Symposium on Naval Hydrodynamics in Fukuoka, Japan, July 2002. COVER STORY C Th [-] K Q S / K Q M [ - ]

Assisting Nozzle Pressure Design

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