Hydrodynamics test apparatus

In all of these tests, flame acceleration was minimal or absent. Acceleration, when it occurred, was entirely due to intrinsic flame instability, for example, hydrodynamic instability (Istratov and Librovich 1969) or instability due to selective diffusion (Markstein 1964). To investigate whether the flame would accelerate when allowed to propagate over greater distances, tests were carried out in an open-sided test apparatus 45 m long (Harris and Wickens 1989). Flame acceleration was found to be no greater than in the balloon experiments (Table 4.1a). 

Vibratory test apparatuses are relatively cheap to build and run, and have low power consumption, while flow rigs are bulky, expensive to build and run, and have high power consumptions but have the advantage that they simulate more closely practical conditions of hydrodynamic cavitation. On the other hand, the damage rate is higher in the vibratory tests than in the... 

Various dissolution test systems have been developed and several of them now enjoy compendial status in pharmacopeias, for example the reciprocating cylinder (United States Pharmacopeia Apparatus 3), the flow-through apparatus [European Pharmacopoeia (Pharm. Eur.) 2.9.3], or the apparatus for transdermal delivery systems, such as the paddle over disc. Hydrodynamic properties of these and other apparatus have been described only sparingly. The paucity of quantitative data related to hydrodynamics of pharmacopeial dissolution testers is lamentable, since well-controllable hydrodynamics are essential to both biopharmaceutical simulations and quality control. Here, we focus the discussion on the paddle and the basket apparatus, since these are the most important and widely used for oral solid dosage forms. A brief treatise on the hydrodynamics of the flow-through apparatus completes this section. 

Hydrodynamics in the upper GI tract contribute to in vivo dissolution. Our ability to forecast dissolution of poorly soluble drugs in vitro depends on our knowledge of and ability to control hydrodynamics as well as other factors influencing dissolution. Provided suitable conditions (apparatus, hydrodynamics, media) are chosen for the dissolution test, it seems possible to predict dissolution limitations to the oral absorption of drugs and to reflect variations in hydrodynamic conditions in the upper GI tract. The fluid volume available for dissolution in the gut lumen, the contact time of the dissolved compound with the absorptive sites, and particle size have been identified as the main hydrodynamic determinants for the absorption of poorly soluble drugs in vivo. The influence of these factors is usually more pronounced than that of the motility pattern or the GI flow rates per se. 

Larger Scale Testing. The standard card gap test (2) is test No. 1 of a series of larger scale tests designed to determine the sensitivity of liquid propellants to hydrodynamic shock. In this test, relative sensitivities of various propellants are determined in terms of the number of 0.01-inch thick cellulose acetate cards required to attenuate a standard shock sufficiently just to prevent initiation in the test sample. When performed according to the exacting conditions of apparatus and procedure, the results are very reproducible from one laboratory to another. However, small variations in the apparatus or procedure can cause major variations in the resulting data, and therefore the test can be considered only relative. A major drawback of the standard test is that it cannot accommodate materials that are volatile under the test condition. At TCC-RMD some special equipment has been developed that allows tests to be made on confined samples at elevated temperature and pressure. 

Scheludko et al. [13,15,73,89,229,230] derived the Reynolds relation in a slightly generalised form and tested it experimentally. The agreement between experiment and theory was very reasonable. More recently, Chan and Horn [231] have used the surface force apparatus (SFA) and found that the Reynolds approach to hydrodynamic lubrication is very successful in describing the drainage of liquid films between smooth solid surfaces. 

All apparatus must be calibrated and the variables standardized and known. Experimental design in dissolution testing to examine residual variation between experimental runs and individual dissolution vessels is feasible. The objective is to minimize errors in experimental set-up, achieved by using mean dissolution times and partial balancing. Lower coefficients of variation occur at higher hydrodynamic intensities. 

The relationship between the hydrodynamics in the GI tract and that in the current available dissolution tests is another factor that must be considered. It has been reported that, provided an appropriate composition is chosen for the dissolution test, the United States Pharmacopeia (USP) paddle apparatus can be used to reflect variations in hydrodynamic conditions in the upper GI tract [67-69]. However, more data are warranted, as this might insert uncertainty into the interpretation of dissolution tests, even when the composition of the GI milieu is well simulated. 

The main merits of the rotating disc apparatus are the well-defined hydrodynamic conditions and constant surface area. These reduce the risk of artefacts in dissolution rate determinations caused by non-ideal test conditions. Furthermore, it is possible to determine an intrinsic dissolution rate and to perform other mechanistic evaluations of the dissolution process. The main limitation of the method is that it is not suitable for drugs that form fragile or porous discs, since it is not possible to maintain a constant surface area. 

Correlation of the in vitro dissolution to the in vivo dissolution is a crucial property of a dissolution test. The major difference in this respect between different apparatus is the hydrodynamic conditions. It has been argued for some of the methods, such as the USP IV flow-through cell or a rotating flask with baffles, that an in vzvo-like situation is created in the... 

Irrespective of the chosen apparatus, the equipment must be set up and handled in a way that both minimises the variability of the dissolution and avoids artefacts. The most common source to such variability or artefacts is hydrodynamic factors, but unwanted chemical reactions or temperature shifts could also occur. Alterations of hydrodynamics, as well as changes of temperature, can both affect the dissolution of a drug substance and the release of a substance from the dosage form. Chemical reactions in the test medium may cause degradation of the drug substance or some formulation excipient which may affect the dissolution, or may lead to misinterpretation of the results. Examples of different sources to variability for the USP apparatuses are summarised below ... 

In comparison with distillation, the most widely used process, knowledge of the fundamentals of Uquid/liquid extraction is limited. A sufficiently accurate description of the hydrodynamics and mass-transfer rates of liquid systems for the design of apparatus is currently not possible for many practical applications.The development of an extraction apparatus generally requires cost-intensive and timepilot plants. Tests with original solutions, for example, from an integrated miniplant, are especially important here (see Section 4.5). 

The use of computer technology and software to compute the hydrodynamic characteristics of eddy eurrents during the development and design of industrial devices. This avoids the need for eostly field tests of gas purification apparatus. 

Experiments were carried out on a purpose built simple hydrodynamic journal bearing test rig (Figure 4 shows a photograph of the apparatus and figure 5 shows a schematic of the loading arm). 

The second strand of recent evidence for slip at the wall is due to Zhu and Granick [7][8]. These authors employed a force balance apparatus to study the hydrodynamic squeeze force between a pair of crossed mica cylinders lubricated by both tetradecane and water. As with the alumina stufaces used by Pit et al. above, the mica surfaces were chemically modified, either by grafting on them a lyophobic monolayer of long chain octadecyl-triethoxysiloxane (OTE) or by adding a surfactant (1-hexadecylamine) to the tetradecane solvent. Contact angle measurements showed that these treatments converted the normally fully-wetted mica surface to one only partially wetted by both test liquids. 

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