1,721,089 research outputs found
Heat transfer prediction in rough cooling channels for liquid rocket engines
No full textAdditive layer manufacturing techniques (ALM) are becoming of increasing interest in the space industry due to their advantages in terms of production costs and times. A peculiar feature resulting from ALM is the large surface roughness, which increases skin friction and heat transfer. This latter aspect can be particularly interesting in the design of regenerative cooling channels as it could increase cooling efficiency. In the field of RANS simulations, the most employed approach to account for wall roughness is based on the equivalent sand grain assumption. Despite this assumption allows to satisfactorily model the friction coefficient, it needs suitable improvements to correctly predict heat transfer at high roughness. To this goal a thermal correction is introduced in the Spalart-Allmaras turbulence model. The implementation of the proposed correction allows to predict realistic values of the convective heat transfer coefficient, in agreement with the experimental results reported in the literature
Analysis of coolant flow and heat transfer in highly rough channels for LRE
No full textThe realization of thrust chambers for the new generation of liquid rocket engines is based on the increasingly popular additive layer manufacturing (ALM) techniques. Such techniques show advantages in terms of design flexibility, production costs and times, but at the same time, if used to manufacture tiny cooling channels lead to high relative surface roughness. In principle, the high roughness of channels is not necessarily unfavorable. In general, it is known to increase heat transfer at the cost of increased pressure loss. However, the correlation between the increase of friction and heat transfer changes at high roughness and the commonly used assumption for heat transfer prediction at low roughness may lead to order of magnitude errors if extended to high roughness channels. Therefore, special corrections must be introduced to have a reliable heat transfer prediction model, such as the one proposed in the present study for circular cross-section channels. The present correction of the Spalart-Allmaras one equation model for the closure of Reynolds averaged Navier-Stokes equations if of easy implementation and allows the user to predict realistic values of the convective heat transfer coefficient, in agreement with the correlations and experimental data reported in the literature
Design and evaluation of aerospike nozzles for an upper stage application
No full textAerospike nozzles are often studied because of their intrinsic self-adaptation behavior to the atmospheric
pressure. Another unique feature of aerospike nozzles is their flexible and effective integration
with the underlying vehicle. This latter aspect, if fully exploited, could be of interest for
both lower stages operating in the atmosphere and for upper stages operating in vacuum. In this paper,
a numerical analysis is carried out of the possible advantages of using an aerospike engine for
a typical upper stage configuration, discussing basic aspects of aerospike and internal expansion
design, clustering and plug truncation length
Chemical Reaction Effects on Wall Heat Flux in Liquid Rocket Thrust Chambers
No full textCooling of thrust chamber walls to allowable solid material temperature induces near wall recombination which may add a non-negligible contribution to the heat transfer from the hot gas to the wall. In this study, the role of near wall recombination is studied by suitable numerical analyses. Numerical results are compared to literature experimental data of wall heat flux in subscale calorimetric thrust chambers for both oxygen/methane and oxygen/hydrogen propellant combination. Then, a parametric analysis is carried out varying chamber pressure, wall temperature and propellant combination. This study highlights that oxygen/methane combustion products are more subject to near wall recombination phenomena. They provide an increase of wall heat flux up to 30% with respect to the frozen value, whereas in the case of oxygen/hydrogen this value does not exceed 10%. Hence, reacting flow simulation is strongly recommended in case of methane fueled thrust chamber analysis, whereas in hydrogen fueled thrust chambers, especially at very high chamber pressure, the difference between reacting and frozen flow model is very small
Separation shock cutoff frequency in dual bell nozzles
No full textThe dual bell nozzle constitutes an option of great interest to improve launcher first stage performance. The internal flow of this nozzle adapts to the external pressure by separating at the wall inflection at low altitude (first operating mode) and by full flowing at high altitude (second operating mode). In such a way the dual bell nozzle operates with a separated flow in the initial portion of the flight trajectory. This characteristic is of important concern since the external flow is neither steady nor axisymmetric, and its coupling with the internal flow separation can cause dangerous side loads. This work presents the results of a time accurate numerical analysis of the effect of an unsteady forcing (imposed by varying the ambient static pressure) on the separation point and on the shock system inside a sub-scale cold-gas dual bell nozzle, operating in the first mode. The spectral analysis of the separation point response to the forcing reveals that the system dynamics is characterized by low-pass filter behavior, with a cutoff frequency around 600 Hz. These results are compared with analogous findings present in literature
Ablative material behavior in oxygen/methane thruster environment
No full textAblative materials represent a low cost and reliable means to insulate rocket engine components from high-temperature, corrosive combustion product environments. Besides their diffuse application in solid rocket nozzles, their use also emerges as a valid alternative in liquid rocket engines. Together with the growing interest in oxygen/methane liquid rocket engines, these materials have gained attention as possible insulator for small upperstage engines or in-space thrusters. In this framework, a validated approach for the study of carbon-based pyrolyzing and non-pyrolyzing materials, together with a novel boundary condition developed to analyze the silica-based material behavior, has been used to numerically reproduce the material response in the highly oxidizing environment generated by the combustion of oxygen and methane. At first, the validation against experimental data of the silica-based material erosion model is presented. Subsequently, the behavior and the response of different ablators in a oxygen/methane environment is numerically investigated for a wide range of operating conditions. Commonly made assumptions in the simulation of the material response are thoroughly analyzed and a critical overview of the results is presented. © 2012 by Alessandro Turchi, Daniele Bianchi, Francesco Nasuti, Renato Marocco
Numerical study of heat transfer in film cooled thrust chambers
No full textFilm cooling as a thermal protection for the walls of liquid rocket engines is studied numerically for hydrogen and methane thrust chamber tests. The aim is to verify the capability of the Reynolds Average Navier-Stokes model to capture the basic characteristics of film-cooled thrust chambers, considering a simplified approach, named pseudo-injector approach, which does not model propellant injection and combustion. This assumption allows a great saving in computational time, in particular when considering 3D simulations. The present study takes its origin from the European Community In-Space Propulsion 1 (ISP-1) program where, among various projects, an experimental campaign has been designed to study the film cooling technique in an oxygen/methane thrust chamber and to provide a database for computational fluid dynamics validation. The results show that the present approach gives good results in terms of heat flux characterization, in particular when dealing with test cases of high chamber pressure. © 2012 by B. Betti, E. Martelli, F. Nasuti, M. Onofri
Flow Separation Response to Unsteady External Disturbances in Dual Bell Nozzles
No full textThe dual bell nozzle belongs to the family of Altitude Compensating Nozzles (ACN), which constitutes an option of great interest to improve launcher first stage performance. The internal flow of this nozzle adapts to the external pressure by separating at the wall inflection at low altitude (first operating mode) and by full flowing at high altitude (second operating mode). Therefore the dual bell nozzle operates with a separated flow at take off and in the initial portion of the flight trajectory. This characteristic is of important concern since the external flow is neither steady nor axisymmetric, and its coupling with the internal flow separation can cause dangerous side loads. This work presents the results of a time accurate numerical analysis of the effect of unsteady ambient pressure on the separation point and on the shock system inside the dual bell nozzle, operating in the first mode. The external computational domain is characterized by an acoustically open downstream boundary. The numerical simulations show that the oscillation of the separation point is very sensitive to frequencies near the acoustic wave characteristic frequencies
Heat Flux Evaluation in Oxygen/Methane Thrust Chambers by RANS Approach
No full textThe demand of a more comprehensive engineering tool for parametric investigations of rocket thrust chambers thermal environment is pushing the improvement of hot-gas side heat transfer prediction tools. In this work, the capability of predicting the heat transfer characteristics of a literature test case for a liquid oxygen / gaseous methane single element combustion chamber is assessed by means of 2D axis-symmetric Reynolds-Averaged Navier-Stokes simulations. A simplied approach is proposed, where combustion is not simulated, but combustion products enter the chamber through a pseudo-injector. The numerical results show a good agreement with the experimental data, except in the near injection region
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