A prediction method for flow in axial compressors

Thesis (M. Ing.) -- University of Stellenbosch, 1995.ENGLISH ABSTRACT: A procedure is presented for predicting the flow through axial compressors. The matrix throughflow equation is transformed to yield an expression of radius dependent on stream function and axial position. The solution of the resultant equation combines the advantages of following streamlines through the calculation domain (as in the streamline curvature method, SCM) with the stability of the matrix throughflow method (MTFM), and is correspondingly called the streamline through flow method (STFM). The predictions of the method were compared to analytical results for a number of inviscid test cases and gave good results. As with the SCM and MTFM, using STFM to predict turbomachinery flows requires empirical models for cascade loss and deflection as well as endwall loss. The off-design loss and deflection model of Howell was used as the basis for a new off-design correlation, H2, valid for axial velocity density ratios (AVDR) of unity. The H2 correlation was developed from the NACA 65-Series database as carpet-plotted by Felix. A simple subcritical Reynolds number correlation for loss and deflection was adapted from a method of Roberts, using inlet blade chord Reynolds number, camber angle, pitch-chord ratio, maximum blade thickness-chord ratio and turbulance factor as parameters. The H2 correlation together with the adapted Roberts correlation gave good predictions of loss and deflection for low-Reynolds number cascade flows at AVDR values of unity. Measurements were taken at the compressor inlet and behind each blade row of a low speed, three stage axial flow compressor at three flowrates: near-design, near-surge and near-choke. The predictions of STFM using Howell's endwall loss models, the modified low Reynolds number correlation and respectively Howell's original off design method and H2 were compared with the experimental results. Howell's method predicted pressure rise to within 3% at design and 10% at off-design, compared to 4% at design and 9% at off-design for the H2 method. The prediction of flow angles for H2 were considerably worse than that of Howell. This was deemed to be caused by AVDR effects. An interim AVDR correlation, dependent on stagger angle, was used together with H2. Choosing values of AVDR for the interim correlation which together with H2 would predict flow angles to match the experimental values, predictions of total pressure rise within 3% at design and 8% at off-design were achieved. As a measure of confidence can be placed in H2 and the modified low Reynolds number correlation, the endwall loss correlation of Howell was determined to be the cause of lack of further gains in accuracy.AFRIKAANSE OPSOMMING: 'n Prosedure vir die voorspel van vloei deur aksiaalkompressors word daargestel. Die matriksdeurvloeivergelyking word getransformeer om 'n uitdrukking van radius, wat afhanklik is van stroomfunksie en aksiale posisie, te gee. Die oplossing van die gevolglike vergelyking span die voordele verbonde aan die volg van stroomlyne deur die berekeningsgebied (soos in the stroomlynkrommingsmetode, SKM) met die stabiliteit van die matriksdeurvloeimetode (MDVM), saam. Dienoorkomstig word die prosedure die stroomlyndeurvloeimetode (SDVM) genoem. Die voorspellings van die metode is met analitiese resultate van 'n aantal nie-viskeuse proefgevalle vergelyk en goeie resultate is behaal. Soos met die SKM en MDVM, genoodsaak die gebruik van SDVM om vloei deur turbomasjiene te voorspel empiriese kaskade verlies- en defleksiemodelle, asook wandverliesmodelle. Die af-ontwerp verlies- en defleksiemodel van Howell is as 'n basis gebruik vir 'n nuwe af-ontwerp korrelasie, H2, wat geldig is vir aksiaalsnelheid-digtheidsverhoudings (ASDV) van een. Die H2 korrelasie is vanuit die NACA 65-Reeks databasis soos deur Felix geplot, ontwikkel. 'n Subkritiese Reynoldsgetal korrelasie vir verlies en defleksie is vanuit 'n metode van Roberts aangepas. Hierdie metode maak gebruik van inlaat lemkoord Reynoldsgetal, krommingshoek, lemspasie-koordverhouding, rnaksimum lemdikte-koordverhouding en turbulensiefaktor as parameters. Die H2 korrelasie, saam met die aangepaste Roberts korrelasie, het goeie voorspellings van verlies en defleksie vir lae Reynoldsgetal kaskadevloeie met ASDV waardes van een gegee. Metings is geneem by die kompressorinlaat en agter elke lemry van 'n lae-spoed, driestadium aksiaalvloeikompressor teen drie massavloeie: naby die ontwerppunt, naby die stollyn en naby die wurglyn. Die voorspellings van SDVM met gebruik van Howell se wandverliesmodelle, die gemodifiseerde lae Reynoldsgetal korrelasie en onderskeidelik Howell se oorspronklike af-ontwerp metode en H2, is met die eksperimentele resultate vergelyk. Howell se metode het drukstyging tot binne 3% by ontwerp en 10% by af-ontwerp voorspel, in vergeleke met 4% by ontwerp en 9% by af-ontwerp vir die H2 metode. Die voorspelling van vloeihoeke deur H2 is heelwat swakker as die van Howell. Dit is as gevolg van ASDV-effekte geag. 'n Interim ASDV korrelasie, afhanklik van lemstelhoek, is saam met H2 gebruik. Deur van ASDV-waardes vir die interim korrelasie te kies wat saam met H2 die eksperimentele vloeihoeke sal voorspel, is totale druk voorspellings binne 3% by ontwerp en 8% by af-ontwerp behaal. Aangesien 'n mate van vertroue in H2 en die aangepasde lae Reynoldsgetal korrelasie geplaas kan word, is die wandverlies korrelasie van Howell uitgesonder as die oorsaak van die gebrek aan verdere verbeterings in akkuraatheid.Maste

Incidence of severe critical events in paediatric anaesthesia (APRICOT): a prospective multicentre observational study in 261 hospitals in Europe

Background Little is known about the incidence of severe critical events in children undergoing general anaesthesia in Europe. We aimed to identify the incidence, nature, and outcome of severe critical events in children undergoing anaesthesia, and the associated potential risk factors. Methods The APRICOT study was a prospective observational multicentre cohort study of children from birth to 15 years of age undergoing elective or urgent anaesthesia for diagnostic or surgical procedures. Children were eligible for inclusion during a 2-week period determined prospectively by each centre. There were 261 participating centres across 33 European countries. The primary endpoint was the occurence of perioperative severe critical events requiring immediate intervention. A severe critical event was defined as the occurrence of respiratory, cardiac, allergic, or neurological complications requiring immediate intervention and that led (or could have led) to major disability or death. This study is registered with ClinicalTrials.gov, number NCT01878760. Findings Between April 1, 2014, and Jan 31, 2015, 31â127 anaesthetic procedures in 30â874 children with a mean age of 6·35 years (SD 4·50) were included. The incidence of perioperative severe critical events was 5·2% (95% CI 5·0â5·5) with an incidence of respiratory critical events of 3·1% (2·9â3·3). Cardiovascular instability occurred in 1·9% (1·7â2·1), with an immediate poor outcome in 5·4% (3·7â7·5) of these cases. The all-cause 30-day in-hospital mortality rate was 10 in 10â000. This was independent of type of anaesthesia. Age (relative risk 0·88, 95% CI 0·86â0·90; p<0·0001), medical history, and physical condition (1·60, 1·40â1·82; p<0·0001) were the major risk factors for a serious critical event. Multivariate analysis revealed evidence for the beneficial effect of years of experience of the most senior anaesthesia team member (0·99, 0·981â0·997; p<0·0048 for respiratory critical events, and 0·98, 0·97â0·99; p=0·0039 for cardiovascular critical events), rather than the type of health institution or providers. Interpretation This study highlights a relatively high rate of severe critical events during the anaesthesia management of children for surgical or diagnostic procedures in Europe, and a large variability in the practice of paediatric anaesthesia. These findings are substantial enough to warrant attention from national, regional, and specialist societies to target education of anaesthesiologists and their teams and implement strategies for quality improvement in paediatric anaesthesia. Funding European Society of Anaesthesiology

Incidence of severe critical events in paediatric anaesthesia (APRICOT): a prospective multicentre observational study in 261 hospitals in Europe

Background Little is known about the incidence of severe critical events in children undergoing general anaesthesia in Europe. We aimed to identify the incidence, nature, and outcome of severe critical events in children undergoing anaesthesia, and the associated potential risk factors. Methods The APRICOT study was a prospective observational multicentre cohort study of children from birth to 15 years of age undergoing elective or urgent anaesthesia for diagnostic or surgical procedures. Children were eligible for inclusion during a 2-week period determined prospectively by each centre. There were 261 participating centres across 33 European countries. The primary endpoint was the occurence of perioperative severe critical events requiring immediate intervention. A severe critical event was defined as the occurrence of respiratory, cardiac, allergic, or neurological complications requiring immediate intervention and that led (or could have led) to major disability or death. This study is registered with ClinicalTrials.gov, number NCT01878760. Findings Between April 1, 2014, and Jan 31, 2015, 31 127 anaesthetic procedures in 30 874 children with a mean age of 6.35 years (SD 4.50) were included. The incidence of perioperative severe critical events was 5.2% (95% CI 5.0-5.5) with an incidence of respiratory critical events of 3.1% (2.9-3.3). Cardiovascular instability occurred in 1.9% (1.7-2.1), with an immediate poor outcome in 5.4% (3.7-7.5) of these cases. The all-cause 30-day in-hospital mortality rate was 10 in 10 000. This was independent of type of anaesthesia. Age (relative risk 0.88, 95% CI 0.86-0.90; p<0.0001), medical history, and physical condition (1.60, 1.40-1.82; p<0.0001) were the major risk factors for a serious critical event. Multivariate analysis revealed evidence for the beneficial effect of years of experience of the most senior anaesthesia team member (0.99, 0.981-0.997; p<0.0048 for respiratory critical events, and 0.98, 0.97-0.99; p=0.0039 for cardiovascular critical events), rather than the type of health institution or providers. Interpretation This study highlights a relatively high rate of severe critical events during the anaesthesia management of children for surgical or diagnostic procedures in Europe, and a large variability in the practice of paediatric anaesthesia. These findings are substantial enough to warrant attention from national, regional, and specialist societies to target education of anaesthesiologists and their teams and implement strategies for quality improvement in paediatric anaesthesia

Incidence of tuberculosis among HIV-infected patients receiving highly active antiretroviral therapy in Europe and North America

Background. We obtained estimates of the incidence of tuberculosis (TB) among patients receiving HAART and identified determinants of the incidence. Methods. We analyzed the incidence of TB during the first 3 years after initiation of HAART among 17,142 treatment-naive, AIDS- free persons starting HAART who were enrolled in 12 cohorts from Europe and North America. We used univariable and multivariable Poisson regression models to identify factors associated with the incidence. Results. During the first 3 years (36,906 person-years), 173 patients developed TB (incidence, 4.69 cases per 1000 person-years). In multivariable analysis, the incidence rate was lower for men who have sex with men, compared with injection drug users (relative rate, 2.46; 95% confidence interval [CI], 1.51-4.01), heterosexuals (relative rate, 2.42; 95% CI, 1.64-3.59), those with other suspected modes of transmission (relative rate, 1.66; 95% CI, 0.91-3.06), and those with a higher CD4(+) count at the time of HAART initiation (relative rate per log(2) cells/mL, 0.87; 95% CI, 0.84-0.91). During 28,846 person-years of follow-up after the first 6 months of HAART, 88 patients developed TB (incidence, 3.1 cases per 1000 person-years of follow-up). In multivariable analyses, a low baseline CD4(+) count (relative rate per log(2) cells/mL, 0.89; 95% CI, 0.83-0.96), 6-month CD4(+) count (relative rate per log(2) cells/mL, 0.90; 95% CI, 0.81-0.99), and a 6-month HIV RNA level 1400 copies/mL (relative rate, 2.21; 95% CI, 1.33-3.67) were significantly associated with the risk of acquiring TB after 6 months of HAART. Conclusion. The level of immunodeficiency at which HAART is initiated and the response to HAART are important determinants of the risk of TB. However, this risk remains appreciable even among those with a good response to HAART, suggesting that other interventions may be needed to control the TB epidemic in the HIV-infected population

Importance of Baseline Prognostic Factors With Increasing Time Since Initiation of Highly Active Antiretroviral Therapy: Collaborative Analysis of Cohorts of HIV-1-Infected Patients

Background: The extent to which the prognosis for AIDS and death of patients initiating highly active antiretroviral therapy (HAART) continues to be affected by their characteristics at the time of initiation (baseline) is unclear. Methods: We analyzed data on 20,379 treatment-naive HIV-1- infected adults who started HAART in 1 of 12 cohort studies in Europe and North America (61,798 person-years of follow-up, 1844 AIDS events, and 1005 deaths). Results: Although baseline CD4 cell count became less prognostic with time, individuals with a baseline CD4 count 350 cells/μL (hazard ratio for AIDS = 2.3, 95% confidence interval [CI]: 1.0 to 2.3; mortality hazard ratio = 2.5, 95% CI: 1.2 to 5.5, 4 to 6 years after starting HAART). Rates of AIDS were persistently higher in individuals who had experienced an AIDS event before starting HAART. Individuals with presumed transmission by means of injection drug use experienced substantially higher rates of AIDS and death than other individuals throughout follow-up (AIDS hazard ratio = 1.6, 95% CI: 0.8 to 3.0; mortality hazard ratio = 3.5, 95% CI: 2.2 to 5.5, 4 to 6 years after starting HAART). Conclusions: Compared with other patient groups, injection drug users and patients with advanced immunodeficiency at baseline experience substantially increased rates of AIDS and death up to 6 years after starting HAART

Variable impact on mortality of AIDS-defining events diagnosed during combination antiretroviral therapy: not all AIDS-defining conditions are created equal.

Abstract Background—The extent to which mortality differs following individual acquired immunodeficiency syndrome (AIDS)–defining events (ADEs) has not been assessed among patients initiating combination antiretroviral therapy. Methods—We analyzed data from 31,620 patients with no prior ADEs who started combination antiretroviral therapy. Cox proportional hazards models were used to estimate mortality hazard ratios for each ADE that occurred in >50 patients, after stratification by cohort and adjustment for sex, HIV transmission group, number of anti-retroviral drugs initiated, regimen, age, date of starting combination antiretroviral therapy, and CD4+ cell count and HIV RNA load at initiation of combination antiretroviral therapy. ADEs that occurred in <50 patients were grouped together to form a “rare ADEs” category. Results—During a median follow-up period of 43 months (interquartile range, 19–70 months), 2880 ADEs were diagnosed in 2262 patients; 1146 patients died. The most common ADEs were esophageal candidiasis (in 360 patients), Pneumocystis jiroveci pneumonia (320 patients), and Kaposi sarcoma (308 patients). The greatest mortality hazard ratio was associated with non- Hodgkin’s lymphoma (hazard ratio, 17.59; 95% confidence interval, 13.84–22.35) and progressive multifocal leukoencephalopathy (hazard ratio, 10.0; 95% confidence interval, 6.70–14.92). Three groups of ADEs were identified on the basis of the ranked hazard ratios with bootstrapped confidence intervals: severe (non-Hodgkin’s lymphoma and progressive multifocal leukoencephalopathy [hazard ratio, 7.26; 95% confidence interval, 5.55–9.48]), moderate (cryptococcosis, cerebral toxoplasmosis, AIDS dementia complex, disseminated Mycobacterium avium complex, and rare ADEs [hazard ratio, 2.35; 95% confidence interval, 1.76–3.13]), and mild (all other ADEs [hazard ratio, 1.47; 95% confidence interval, 1.08–2.00]). Conclusions—In the combination antiretroviral therapy era, mortality rates subsequent to an ADE depend on the specific diagnosis. The proposed classification of ADEs may be useful in clinical end point trials, prognostic studies, and patient management

Variable impact on mortality of AIDS-defining events diagnosed during combination antiretroviral therapy: not all AIDS-defining conditions are created equal

Abstract Background—The extent to which mortality differs following individual acquired immunodeficiency syndrome (AIDS)–defining events (ADEs) has not been assessed among patients initiating combination antiretroviral therapy. Methods—We analyzed data from 31,620 patients with no prior ADEs who started combination antiretroviral therapy. Cox proportional hazards models were used to estimate mortality hazard ratios for each ADE that occurred in >50 patients, after stratification by cohort and adjustment for sex, HIV transmission group, number of anti-retroviral drugs initiated, regimen, age, date of starting combination antiretroviral therapy, and CD4+ cell count and HIV RNA load at initiation of combination antiretroviral therapy. ADEs that occurred in <50 patients were grouped together to form a “rare ADEs” category. Results—During a median follow-up period of 43 months (interquartile range, 19–70 months), 2880 ADEs were diagnosed in 2262 patients; 1146 patients died. The most common ADEs were esophageal candidiasis (in 360 patients), Pneumocystis jiroveci pneumonia (320 patients), and Kaposi sarcoma (308 patients). The greatest mortality hazard ratio was associated with non- Hodgkin’s lymphoma (hazard ratio, 17.59; 95% confidence interval, 13.84–22.35) and progressive multifocal leukoencephalopathy (hazard ratio, 10.0; 95% confidence interval, 6.70–14.92). Three groups of ADEs were identified on the basis of the ranked hazard ratios with bootstrapped confidence intervals: severe (non-Hodgkin’s lymphoma and progressive multifocal leukoencephalopathy [hazard ratio, 7.26; 95% confidence interval, 5.55–9.48]), moderate (cryptococcosis, cerebral toxoplasmosis, AIDS dementia complex, disseminated Mycobacterium avium complex, and rare ADEs [hazard ratio, 2.35; 95% confidence interval, 1.76–3.13]), and mild (all other ADEs [hazard ratio, 1.47; 95% confidence interval, 1.08–2.00]). Conclusions—In the combination antiretroviral therapy era, mortality rates subsequent to an ADE depend on the specific diagnosis. The proposed classification of ADEs may be useful in clinical end point trials, prognostic studies, and patient management