Volker Kottke

Effect of baffle spacing on pressure drop and local heat transfer in shell-and-tube heat exchangers for staggered tube arrangement

Local heat transfer and pressure drop on the shell side of shell-and-tube heat exchangers with segmental baffles were investigated for different baffle spacings. The distributions of the local heat transfer coefficients on each tube surface within a fully developed baffle compartment were determined and visualized by means of mass transfer measurements. Per-tube, per-row and per-compartment average heat transfer coefficients were drawn from the local values. The local pressure measurements allow the determination of the shell-side flow distributions. For same Reynolds number, the pressure drop and average heat transfer are increased by an increased baffle spacing due to a reduced leakage through the baffle-shell clearance. The experimental results were compared with literature values.

Visualization and determination of local heat transfer coefficients in shell-and-tube heat exchangers for staggered tube arrangement by mass transfer measurements

Local heat transfer coefficients on the outer surface of tubes in shell-and-tube heat exchangers with staggered tube arrangement are visualized and determined from mass transfer measurements. The mass transfer experiments are carried out using a technique based on absorption, chemical and coupled colour reaction. Local mass transfer coefficients are measured for fully developed flow conditions on each tube surface. These coefficients are transformed to heat transfer coefficients by employing the analogy between heat and mass transfer. The averaged heat transfer coefficients and the pressure drop are compared with the predictions from the literature.

Sinusoidal wavy channels with Taylor-Goertler vortices

Abstract In the concave part of sinusoidal wavy ducts, a flow instability can be observed, leading to longitudinal vortices comparable to the Goertler instability at concave walls or the Dean instability in tube flows. Near-wall flow phenomena are visualized by the visualization of the local mass transfer. The pressure losses are evaluated in a sinusoidal wavy duct with constant wavelength λ/ a , amplitude of the wavy wall a = 1.825 mm, duct width B = 150 mm, and duct length L = 400 mm for various duct heights s and a range of Reynolds numbers. A stability diagram describes the existing range of longitudinal vortices. It is shown that the wavelength ψ V of the vortices changes significantly depending on the spacing of the duct.

Comparison of heat and mass transfer in different heat exchanger geometries with corrugated walls

Abstract In systematic comparative analyses heat exchangers with corrugated walls are analysed. The wall corrugation is generated by sinusoidally shaped walls as well as by crosswise corrugated cylinders of different cross-section on plane walls like those being used as spacers in membrane technology. Corrugated structures in crosswise orientation are composed of passages formed by layers of corrugated structures with opposite orientation of the corrugation lines. The main geometrical parameters of such structures are the inclination angle ϕ , the wavelength λ , the amplitude a and the shape of the corrugation. The effect of different geometrical parameters of these corrugated structures on the local and integral heat and mass transfer are discussed. It is shown, that the flow phenomena caused by the different geometries are of significant influence on the homogeneity and on the quantity of the local heat and mass transfer as well as on the pressure drop. Independent of the sinusoidal or cylindrical surface structure the heat and mass transfer as well as the pressure losses are fixed by basic flow characteristics. Here three different flow types can be separated, depending on the geometry and flow parameters. For applications of heat transfer in fluids with particulates the use of corrugated walls in the free-flow arrangement becomes more interesting. Therefore results for this heat exchanger geometry are presented, where the transition from laminar to turbulent flow range is dominated by the Gortler–Dean instability. Besides a detailed flow analysis, comparison of the local heat and mass transfer and the pressure losses for these geometries are presented. The measurements were made at the same flow velocity, channel height and channel width as the crosswise corrugated structures. A direct and accurate comparison of the different structures is thus possible.

The Influence of Humidity, Hydrogen Peroxide Concentration, and Condensation on the Inactivation of Geobacillus stearothermophilus Spores with Hydrogen Peroxide Vapor

The study presented here examined the factors influencing the effectiveness of surface decontamination with hydrogen peroxide vapor. The impact of relative humidity and hydrogen peroxide gas concentrations was investigated and compared to a dew point analysis of these various sterilant atmospheres. For this purpose, a series of different H2O2 decontamination cycles were developed and tested for antimicrobial effectiveness using biological indicators inoculated with greater than 106 spores of Geobacillus stearothermophilus. The results indicate that an increasing concentration of hydrogen peroxide in the gas phase and higher humidity levels result in a faster inactivation of the test organisms. The higher the H2O2 gas phase concentration was, the more independent the inactivation effect from the humidity level. At lower H2O2 concentrations, the same kill was achieved with higher humidity. Subvisible condensation was found to be necessary for short inactivation times, but condensation in the visible range did not further enhance the sporicidal activity. The molecular deposition of water and hydrogen peroxide on the target surface represents the determining factor for microbial inactivation, whereas the hydrogen peroxide concentration in the gas phase is of secondary importance.

Analysis of local shellside heat and mass transfer in the shell-and-tube heat exchanger with disc-and-doughnut baffles

A mass transfer measuring technique is used to visualize and determine the shellside local heat transfer coefficients at each tube in two representative baffle compartments of a shell-and-tube heat exchanger with disc-and-doughnut baffles. The fluid flow adjacent to the tube is analysed and the heat transfer in the zones of separated flow discussed. The shellside flow distribution is determined through the measurements of the local pressure drop in the baffle-tube and baffle-shell clearances. Compared to the single-segmental baffle, the disc-and-doughnut baffles have a higher effectiveness of heat transfer to pressure drop. This investigation presents also per-tube and per-compartment averaged heat transfer coefficients.

Crossflow filtration as a new method of wet classification of ultrafine particles

Abstract The particle layer build-up during crossflow microfiltration (CMF) has been examined experimentally. To gain more information than only the time dependent filtrate flux, specially designed flat duct modules have been developed. These modules provide defined flow conditions and allow an easy removal of the particle layer covered membrane. The particle layer thickness and topography, the specific mass flow rate and the particle size distribution were recorded. The deposition process was examined in turbulent and in laminar flow as well as for particles having a negligible density difference compared to the liquid. These results led to a new process of wet classification, that takes advantage of the primary disadvantegeous effect of selective particle deposition. The aim of the new process is to win the particle layer as fine product. The process is subdivided in two steps. In the first step, the fines deposit on the membrane and then are discharged by back-flushing in the second step. A new pilot plant with tubular membranes shows the transferability of the flat duct experiments and proves the feasibility of the two process steps. The influence of the determining parameters flow velocity, filtrate flux and particle concentration has been studied.

Fouling in Membrane Apparatus: The Mechanisms of Particle Deposition

A detailed understanding of the mechanisms of particle-deposition and particle-removal during Crossflow Microfiltration (CMF) is essential for an improved design of membrane modules used in separation processes. The influence of the governing parameters filtrate flow rate, flow velocity and particle concentration on the local particle deposition is analysed both at constant transmembrane pressure and constant filtrate flow rate. For the investigation of the particle layer build-up under well-defined boundary and entrance conditions, new flat gap membrane modules are developed. Using these modules, a homogeneous plug flow profile at the entrance of the module is generated. The thickness of the cake layer received for different boundary conditions is determined by means of a stepping motor-driven laser-autofocus-sensor with high resolution. These thickness measurements allow a precise determination of the cake layers surface topography. The net mass flow of every particle size fraction at the membrane during different time intervals and at different positions is determined. The investigations provide quantitative results of the selective particle deposition and allow an evaluation of existing models. Depending on the particle size distribution and the flow parameters, a particle-free membrane can be achieved.

Local heat transfer in the first baffle compartment of the shell-and-tube heat exchangers for staggered tube arrangement

Abstract A mass transfer measuring method based on absorption, chemical and coupled colour reaction is used to visualize and determine the shell side local heat transfer in the first baffle compartment of shell-and-tube heat exchangers with segmental baffles for staggered tube arrangement. Local mass transfer coefficients were transformed into heat transfer coefficients by using the analogy between heat and mass transfer. The local, per-tube and integral heat transfer coefficient distributions are presented.

Hydrogen Peroxide Vapor Penetration into Small Cavities during Low-Temperature Decontamination Cycles

IntroductionThe suitability of vapor-phase hydrogen peroxide for the decontamination of different-sized narrow cavities and complex geometrical structures were investigated in this paper.MethodsA cavity test block was used, and cavities made from different materials were tested with variable entrance heights and cavity depths. At the end of each cavity, biological indicators were exposed as a microbiological challenge for vapor-phase hydrogen peroxide penetration. Within this study, the test block with the biological indicators was subjected to different decontamination cycles in a production isolator. Inoculation level, cycle length, hydrogen peroxide, and water concentration were varied.ResultsThe ratio of cavity entrance height to depth was found to be critical for decontamination success by biological indicators exposed inside the cavities. The higher the ratio, the more spores could be inactivated. Inactivation is also effected by exposure time and hydrogen peroxide concentration.ConclusionThe results indicate that the entrance height of the cavities should not be smaller than 6 mm and the cavity depth should not exceed 30 mm. If smaller cavities cannot be avoided, high peroxide concentration (800 ppm) and prolonged cycle times were shown to significantly enhance the penetration into dead-ended cavities under diffusive conditions.