86,627 research outputs found
Coupled numerical simulation of wall heat conduction and coolant flow in liquid rocket engines
No full textCoolant-flow modeling in regeneratively cooled rocket engines fed with turbo-machinery is a challenging task because of the high wall temperature gradient, the high Reynolds number, the high aspect ratio of the channel cross section and the heat transfer coupling with the hot-gas flow and the solid material. In this study the effect of wall heat conduction on the coolant flow is analyzed by means of coupled computations between a validated Reynolds Averaged Navier-Stokes Equations solver for the coolant flow-field and a Fourier's equation solver for the thermal conduction in the solid material. Computations of supercritical-hydrogen flow in a straight channel with and without coupling with the solid material are performed and compared in order to understand the role played by the coupling on the coolant flow evolution. Finally, the whole cooling circuit of the Space Shuttle Main Engine Main Combustion Chamber is analyzed in detail; in this case, the hotgas flow is described by means of experimental data of the heat transfer coefficient while the wall structure and the coolant flow behaviors are described by the coupled procedure presented in this work. © 2011 by M. Pizzarelli, F. Nasuti and M. Onofri
Coupled Heat Transfer Analysis in Regeneratively Cooled Thrust Chambers
No full textA computational procedure able to describe the coupled hot-gas/wall/coolant environment that occurs in most liquid rocket engines and to provide a quick and reliable prediction of thrust-chamber wall temperature and heat flux as well as coolant-flow characteristics, like pressure drop and temperature gain in the regenerative circuit is presented and demonstrated. The coupled analysis is performed by means of an accurate CFD solver of the Reynolds-Averaged Navier-Stokes equations for the hot-gas flow and a simplified quasi-2D approach, which widely relies on semi-empirical relations, to study the problem of coolant flow and wall structure heat transfer in the cooling channels. Coupled computations of the Space Shuttle Main Engine Main Combustion Chamber are performed and compared with available literature data. Results show a reasonable agreement in terms of coolant pressure drop and temperature gain with nominal data, whereas the computed wall temperature peak is quite closer to hot-firing data than to the nominal value. © 2012 by B. Betti, M. Pizzarelli, F. Nasuti
Pseudo-boiling and heat transfer deterioration while heating supercritical liquid rocket engine propellants
No full textHeating of liquid propellants used as the coolant in rocket engines may lead to undesired phenomena such as pseudo-boiling or heat transfer deterioration under specific conditions. This can be an issue for propellants characterized at the same time by relatively low critical pressure and temperature. Light hydrocarbons, as for instance methane, belong to this family. In the present paper, a critical review is made of the main results obtained by Authors and their coworkers for the present application. Focus is on the correlations and trends inferred by their numerical simulations mainly carried out considering methane as the coolant, perhaps the most challenging one
Analysis on the effect of channel aspect ratio on rocket thermal behavior
No full textA trade-off analysis is performed on a test case representative of the cooling system of 1 MN thrust class oxygen/hydrogen liquid rocket engine. The aim of the analysis is to find the channel aspect ratio that maximizes the heat extracted from the hot-gas, for a given coolant pressure drop and hot-gas side wall temperature. The analysis requires many cooling channel flow calculations which are performed by means of a simplified model, referred to as quasi-2D, and an accurate conjugate heat transfer model based on numerical integration of the Navier-Stokes and Fourier's Equations. Both models are able to describe the whole cooling device composed by the coolant and the solid domain, which is exposed the hot-gas, with different computational time and level of details. The fast quasi-2D approach is used to select channel geometries showing the same pressure loss. Discussion is made on results obtained with the more accurate CHT model. Results identify, for the selected test case, an ideal aspect ratio which optimizes cooling performance at large values of channel aspect ratio © 2012 by M. Pizzarelli, F. Nasuti, M. Onofri
Oxygen–methane rocket thrust chambers: Review of heat transfer experimental studies
No full textNowadays oxygen–methane propulsion is considered of great potential for the next generation of operational rockets. In the development of a new liquid rocket engine with high-performance, an essential assessment is the thermal behavior of the thrust chamber. In fact, the flow of combustion gas inside the thrust chamber induces a level of wall temperature and heat flux such as to limit the life of this component. Furthermore, the heat transfer within the thrust chamber directly impacts the performance of the rocket engine. In the present study, a thorough survey of the open literature concerning the experimental activities focused on the heat transfer within oxygen–methane thrust chambers is carried out in order to identify the most consolidated results and the aspects that would require further investigations. The results are divided into three main categories: hot-gas side heat transfer; coolant heat transfer and wall compatibility; thrust chamber life
IRON-REPLETE HAEMODIALYSIS PATIENTS DO NOT REQUIRE HIGHER ERYTHROPOIETIN DOSAGES WHEN CONVERTING FROM SUBCUTANEOUS TO INTRAVENOUS ADMINISTRATION. RESULTS OF THE ITALIAN STUDY ON ERYTHROPOIETIN CONVERTING (ISEC).
No full textIron-replete hemodialysis patients do not require higher EPO dosages when converting from subcutaneous to intravenous administration: results of the Italian Study on Erythropoietin Converting (ISEC).
No full textBACKGROUND:
Previous studies reported significant increases in epoetin dosages when converting hemodialysis patients from subcutaneous (SC) to intravenous (IV) administration. More recent studies that corrected for iron deficiency found a much lower, if any, increase in epoetin dosage and/or decrease in hemoglobin (Hb) level after conversion from SC to IV epoetin administration. Therefore, the matter is still open for debate.
METHODS:
This multicenter observational study evaluated stable hemodialysis patients without iron deficiency who had a stable SC epoetin dosage and Hb level of 10 g/dL or greater (> or =100 g/L) at the time of study enrollment. Data for epoetin dosage, anemia, and inflammatory markers were collected retrospectively during the last 6 months of SC epoetin treatment and prospectively for 6 months after conversion to IV administration. The primary efficacy assessment was difference in Hb levels and epoetin dosages between patients administered epoetin SC and IV. Changes in values for iron stores, C-reactive protein, intact parathyroid hormone, and albumin were monitored as control parameters.
RESULTS:
Data were analyzed for 262 hemodialysis patients from 6 Italian centers. Overall, mean Hb levels were similar with SC and IV epoetin administration (11.49 g/dL [114.9 g/L] and 11.44 g/dL [114.4 g/L]). Mean epoetin dosages also were similar with SC and IV administration (7,185 and 7,270 IU/wk). In patients requiring epoetin dosages of 12,000 IU/wk or greater at study entry, mean dosages tended to decrease after conversion to IV administration. There were no significant changes in control parameters.
CONCLUSION:
Conversion from SC to IV epoetin administration did not result in changes in Hb levels or epoetin dosage requirements in iron-replete hemodialysis patients
A Simplified Thermal Analysis Model for Regeneratively Cooled Rocket Engine Thrust Chambers and Its Calibration with Experimental Data
No full textAn essential part of the design of a liquid rocket engine is the thermal analysis of the thrust chamber, which is a component whose operative life is limited by the maximum allowable wall temperature and heat flux. A simplified steady-state thermal analysis model for regeneratively cooled rocket engine thrust chambers is presented. The model is based on semi-empirical correlations for the hot-gas and coolant convective heat transfer and on an original multi-zone approach for the wall conduction. The hot-gas heat transfer is calibrated with experimental data taken from an additively manufactured water-cooled nozzle that is connected to a combustion chamber either fed with decomposed hydrogen peroxide or decomposed hydrogen peroxide and automotive diesel. The thrust chamber (i.e., combustion chamber and nozzle) is designed to produce about 450 N of thrust when operating with a chamber pressure of 11 bar. For this application, the calibrated model predicts the total wall heat transfer rate very accurately and the temperature distribution within the wall structure with an uncertainty of a few tens of kelvins. This level of accuracy can be considered more than adequate for the design, and generally for engineering-type thermal analysis, of similar thrust chambers
Cardiovascular stability during haemodialysis, haemofiltration and haemodiafiltration
No full textSeveral comparative studies have claimed that procedures based substantially or exclusively on pressure-driven water-solute transport, such as haemodiafiltration or haemofiltration, afford better protection of the cardiovascular tolerance to fluid removal than conventional haemodialysis. During each depurative modality, several factors are set in motion that might impact, each in its own right, upon the haemodynamic response to fluid withdrawal. To explore the haemodynamic effect of each of them singularly, one needs to keep all other components unvaried. However, this is very difficult to accomplish. For instance, to confirm the alleged greater protection of cardiovascular stability by pure convection vs diffusion, one needs to keep unvaried all the other factors potentially affecting haemodynamic tolerance, i.e. the rate of body fluid removal, the membrane, the buffer, the blood temperature in the extracorporeal circuit, depuration efficiency, the sodium balance, the fluid sterility and so on. Such studies are still awaited. However, clinical trials published to date have not resolved the question of whether haemofiltration and haemodiafiltration provide a better haemodynamic tolerance to fluid removal. If we limit our consideration to controlled trials only, most prospective studies have adopted a cross-over design implemented on very small patient samples and for very short periods. Such an approach is liable to generate misleading results because the incidence of dialysis hypotension often fluctuates from time to time. Owing to such fluctuations, results can be strongly affected by the 'order effect' of the cross-over from one technique to the other. The negative results provided by parallel comparisons of procedures should be taken with caution because patients samples did not include a suitable proportion of unstable patients
Predictivity of survival according to different equations for estimating renal function in community-dwelling elderly subjects
No full textBackground. Detection of subjects with early chronic kidney disease (CKD) is important because some will progress up to stage 5 CKD, and most are at high risk of cardiovascular morbidity and mortality. While validity and precision of estimated glomerular filtration rate (eGFR) equations in tracking true GFR have been repeatedly investigated, their prognostic performance for mortality has not been hitherto compared. This is especially relevant in an elderly population in whom the risk of death is far more common than progression. Methods. We analysed data of participants in the InCHIANTI study, a community-based cohort study of older adults. Twenty-four-hour creatinine clearance (Ccr), Cockcroft-Gault (C-G) and Modification of Diet in Renal Disease (MDRD)-derived equations (six and four input variables) were calculated at enrolment (1998-2000), and all-cause mortality and cardiovascular mortality were prospectively ascertained by Cox regression over a 6-year follow-up. Results. Of the 1270 participants, 942 (mean age 75 years) had complete data for this study. The mean renal function ranged from 77 ml/min/1.73 m(2) by Ccr to 64 ml/min/1.73 m(2) by C-G. Comparisons among equations using K/DOQI staging highlight relevant mismatches, with a prevalence of CKD ranging from 22% (MDRD-4) to 40% (C-G). Reduced renal function was a strong independent predictor of death. In a Cox model-adjusted for demographics, physical activity, comorbidities, proteinuria and inflammatory parameters-participants with Ccr 60-90 ml/min/1.73 m(2) and Ccr 90 ml/min/1.73 m(2). For the C-G, the group with values 90 ml/min/1.73 m(2) (HR 2.59, 95% CI: 1.13-5.91). The classification based on the MDRD formulae did not provide any significant prognostic information. The adjusted risk of all-cause mortality followed a similar pattern when Ccr and estimating equations were introduced as continuous variables or dichotomized as higher or lower than 60 ml/min. C-G was the best prognostic indicator of cardiovascular mortality. Possibly, Ccr and C-G are better prognostic indicators than MDRD-derived equations because they incorporate a stronger effect of age. Conclusions. In a South-European elderly population, the prevalence of CKD is high and varies widely according to the method adopted to estimate GFR. Researchers and clinicians who want to capture the prognostic information on mortality related to kidney function should use the Ccr or C-G formula and not MDRD equations. These results highlight the importance of strategies for early detection and clinical management of CKD in elderly subjects
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