Studies on free stream turbulence as related to gas turbine heat transfer. A review of authors' past work and future implications.

A review of the past work done on free stream turbulence (FST) as applied to gas turbine heat transfer and its implications for future studies are presented. It is a comprehensive approach to the results of many individual studies in order to derive the general conclusions that could be inferred from all rather than discussing the results of each individual study. Three experimental and four modeling studies are reviewed. The first study was on prediction of heat transfer for film cooled gas turbine blades. An injection model was devised and used along with a 2-D low Reynolds number k-epsilon model of turbulence for the calculations. Reasonable predictions of heat transfer coefficients were obtained for turbulence intensity levels up to 7%. Following this modeling study a series of experimental studies were undertaken. The objective of these studies was to gain a fundamental understanding of mechanisms through which FST augments the surface heat transfer. Experiments were carried out in the boundary layer and in the free stream downstream of a gas turbine combustor simulator, which produced initial FST levels of 25.7% and large length scales (About 5-10 cm for a boundary layer 4-5 cm thick). This result showed that one possible mechanism through which FST caused an increase in heat transfer is by increasing the number of ejection events. In a number of modeling studies several well-known k-epsilon models were compared for their predictive capability of heat transfer and skin friction coefficients under moderate and high FST. Two data sets, one with moderate levels of FST (about 7%) and one with high levels of FST (about 25%) were used for this purpose. Although the models did fine in their predictions of cases with no FST (baseline cases) they failed one by one as FST levels were increased. Under high FST (25.7% initial intensity) predictions of Stanton number were between 35-100% in error compared to the measured values. Later a new additional production term indicating the interaction between the turbulent kinetic energy (TKE) and mean velocity gradients was introduced into the TKE equation. The predicted results of skin friction coefficient and Stanton number were excellent both in moderate and high FST cases. In fact these model also gave good predictions of TKE profiles whereas earlier unmodified models did not predict the correct TKE profiles even under moderate turbulence intensities. Although this new production term seems to achieve the purpose, it is the authors' belief that it is diffusion term of the TKE equation, which needs to be modified in order to fit the physical events in high FST boundary layer flows. The results of these studies are currently being used to come up with new diffusion model for the TKE equation.

In vitro characterization of the erythrocyte distribution of methazolamide: a model of erythrocyte transport and binding kinetics.

The rate and extent of binding of methazolamide to human erythrocytes was studied in vitro. All experiments were carried out at physiological temperature (37 C) and pH (7.4). Methazolamide (MTZ) buffer concentrations were analyzed by HPLC. Distributional equilibrium between buffer and washed red blood cells was achieved after 1 hr. Results of equilibrium studies were consistent with two classes of binding sites for MTZ within the erythrocyte: a low affinity, high capacity site (CA-I) and a high affinity, low capacity site (CA-II). A two-binding site model was fitted to experimental data generating estimates for binding parameters Ka1 (0.0017 +/- 0.00022 microM-1) nM1 (636 +/- 5.23 microM), Ka2(0.46 +/- 0.0083 microM-1), and nM2(80.9 +/- 0.389 microM). Based upon these findings, kinetic studies were performed in order to characterize the rate of drug distribution. The rate of erythrocyte uptake of MTZ was mathematically modeled using a series of differential equations describing drug diffusion across the red blood cell membrane and subsequent complexation with intracellular binding sites. The model assumed that penetration of MTZ into the red blood cells was passive but drug binding to the carbonic anhydrase isozymes was not instantaneous. Using a novel curve fitting technique, parameter estimates of RBC membrane permeability (0.0102 +/- 0.000618 cm/min), and binding rate constants k-1(0.254 +/- 0.0213 min-1), k1 (0.0022 +/- 0.00020 ml/microgram-min), k-2(1.59 +/- 0.0358 min-1), and k2(3.1 +/- 0.035 ml/microgram-min) were obtained. The model characterized the observed biphasic decline of MTZ buffer concentrations over time and may help explain the prolonged residence of MTZ in vivo.

Blood disposition and urinary excretion kinetics of methazolamide following oral administration to human subjects.

The pharmacokinetic disposition of methazolamide (MTZ) was studied in five healthy volunteers following administration of a single oral dose. Drug concentrations in blood, plasma, and urine were measured by HPLC. Over the range of plasma concentrations observed in vivo, MTZ free fraction (measured by ultrafiltration) was 0.28. Being a carbonic anhydrase inhibitor, MTZ would be expected to distribute into, and be sequestered by, red blood cells. For this reason, MTZ disposition was characterized utilizing blood concentrations as the reference. Using a two-compartment model, a series of differential equations were simultaneously fitted to blood concentrations and urinary excretion data generating estimates for k10 (0.035 +/- 0.019 h(-1)), k12 (0.200 +/- 0.036 h(-1)), k21 (0.077 +/- 0.046 h(-1)), k(a) (0.304 +/- 0.064 h(-1)), Vc (1.1 +/- 0.18 L) and f(r) (fraction excreted renally, 0.61 +/- 0.14). Total blood clearance was 0.037 +/- 0.020 L h(-1). The model estimate of elimination half-life (126 +/- 61 h) was consistent with drug binding to a high affinity carbonic anhydrase isozyme in the erythocyte. Estimates of MTZ renal clearance and renal excretion ratio were 0.021 +/- 0.010 L h(-1) and 0.16 +/- 0.06, respectively. Overall, the prolonged elimination of MTZ from the blood is the result of extensive erythrocyte distribution and tubular reabsorption by the kidney.

Determination of methazolamide concentrations in human biological fluids using high performance liquid chromatography.

Methazolamide is a carbonic anhydrase inhibitor used to treat glaucoma. In vivo, methazolamide readily distributes into red blood cells. Therefore, both blood and plasma concentration data are needed in order to characterize the pharmacokinetics of methazolamide. In the present study, an analytical method using high performance liquid chromatography was validated for determination of methazolamide concentrations in several biological fluids. Through slight modification of a previously reported method for acetazolamide, another carbonic anhydrase inhibitor, methazolamide was readily quantitated in whole blood, plasma and urine. Sample preparation involved liquid-liquid extraction with ethyl acetate followed by a washing step using phosphate buffer (pH 8.0). After back extraction into glycine buffer (pH 10.0), samples were then washed with ether and injected onto the chromatograph. Chromatography was performed using a C-18, 5 microns reverse-phase column with UV detection at a wavelength of 285 nm. Mobile phase consisted of 0.05 M sodium acetate (pH 4.0) and acetonitrile (20%). The assay was validated over two standard concentration ranges from 1 to 100 micrograms ml-1, concentrations reflective of those expected in vivo, Calibration curves were linear for all biological fluids and coefficients of variation for interday and intraday reproducibility studies were less than 8% (range 3.1-7.9%). The method was used to measure methazolamide concentrations in blood, plasma and urine following oral administration to five human subjects.

Application of a first-pass effect model to characterize the pharmacokinetic disposition of venlafaxine after oral administration to human subjects.

Venlafaxine (VEN), a drug used in the treatment of depression, undergoes significant first-pass metabolism after oral dosing to O-desmethylvenlafaxine (ODV), a metabolite with comparable therapeutic activity to that of parent drug. The pharmacokinetic disposition of VEN was characterized using a "first-pass" model that incorporates a presystemic compartment (liver) to account for the first-pass metabolism of VEN to ODV. A series of differential equations were simultaneously fitted to plasma concentrations of parent and metabolite. A good fit of the model to observed data was demonstrated, generating estimates for the following parameters: ka (1.31 +/- 0.009 hr-1), VVEN (252 +/- 87.6 liters), CLint (65.8 +/- 39.7 liters/hr), RL (liver:plasma partition coefficient, 29.6 +/- 18. 3), VODV (181 +/- 84.1 liters), and CLODV (23.5 +/- 12.5 liters/hr). Parameter estimates correlated closely with those obtained through noncompartmental methods. These results indicate that the time-course disposition of a compound undergoing first-pass hepatic metabolism after oral dosing can be successfully modeled.

ZBP-89, a Krüppel-like zinc finger protein, inhibits epidermal growth factor induction of the gastrin promoter.

We have shown previously that a GC-rich element (GGGGCGGGGTGGGGGG) conferring epidermal growth factor (EGF) responsiveness to the human gastrin promoter binds Sp1 and additional undefined complexes. A rat GH4 cell line expression library was screened by using a multimer of the gastrin EGF response element, and three overlapping cDNA clones were identified. The full-length rat cDNA encoded an 89-kDa zinc finger protein (ZBP-89) that was 89% identical to a 49-kDa human factor, ht(beta), that binds a GTGGG/CACCC element in T-cell receptor promoters. The conservation of amino acids between the zinc fingers indicates that ZBP-89 is a member of the C2H2 zinc finger family subclass typified by the Drosophila Krüppel protein. ZBP-89 is ubiquitously expressed in normal adult tissues. It binds specifically to the gastrin EGF response element and inhibits EGF induction of the gastrin promoter. Collectively, these results demonstrate that ZBP-89 functions as a repressor of basal and inducible expression of the gastrin gene.