Experimental study of low-frequency oscillations and large-scale circulations in turbulent mixed convection

The formation and dynamics of large-scale circulations in forced and mixed convection has been studied at ambient and elevated fluid pressure by means of particle image velocimetry and temperature measurements. The study has been conducted in two rectangular containers of the same shape and aspect ratios of Cxz = 1 and Cyz = 5. For the measurements at high fluid pressure the dimensions of the cell have been scaled down by a factor of 5. Air with Pr = 0.7 has been used as fluid in both configurations. Forced convection has been investigated at Re = 1.01 x 104 and mixed convection has been studied at Ar = 3.3, Re = 1.01 x 10 up4 and Ra = 2.4 x 10 up8. In this configuration low-frequency oscillations in the heat transfer between the inlet and outlet have been found for mixed convection. Instantaneous velocity vector fields obtained from particle image velocimetry have been analysed using proper orthogonal decomposition and an algorithm to detect the core and the core centre position of large-scale circulations

Experimentelle Untersuchung zum thermischen Komfort im Grenzbereich des Luftzugempfindens in einer generischen Fahrzeugkabine

Ob auf dem Weg zur Arbeit oder in den Urlaub – in den heutigen Industriestaaten verbringt der Mensch einen nicht unwesentlichen Teil seiner Zeit im Fahrzeug. Nicht zuletzt deswegen und aufgrund des gesteigerten Wettbewerbs ist der thermische Komfort im Fahrzeug ein mitentscheidendes Argument für die Profilierung im Wettbewerb der Fahrzeughersteller. Für die Auslegung einer Klimatisierung im PKW ist es das Ziel thermische Bedingungen herzustellen, die von möglichst allen Passagieren als behaglich wahrgenommen werden. In diesem Zusammenhang stellt sich die Frage, wie man den thermischen Komfort in einem Fahrzeug vergleichbar bewerten kann. Hierzu gibt es eine Vielzahl von Methoden und Normen zur Charakterisierung und Bewertung des thermischen Komforts in PKW. Jedoch zeigen Untersuchungen, dass die Normen den thermischen Komfort im Grenzbereich des Zugluftempfindens nur unvollständig beschreiben. Das Ziel dieses Forschungsvorhabens ist daher die Untersuchung der Auswirkung von Zugluft auf den thermischen Komfort in Abhängigkeit der folgenden Einflussgrößen: Lufttemperatur, Strahlung, Luftmassenstrom, Luftgeschwindigkeit und Turbulenzgrad. Hierzu wurden die objektiven Komfortparameter in einer generischen Fahrzeugkabine mithilfe eines thermischen Menschmodells bestimmt und basierend auf der Methode der Äquivalenttemperatur nach EN ISO14505-2 bewertet. Die Ergebnisse der Untersuchung sollen mit den aktuellen Normen verglichen werden. Im Zuge dessen soll außerdem geprüft werden, inwieweit eine Anpassung bestehender Normen unter Berücksichtigung des Luftzugempfindens möglich und sinnvoll ist. Auf der Fachtagung werden wir die Ergebnisse einer Teilstudie zur Untersuchung des thermischen Komforts im Grenzbereich des Zugluftempfindens als Funktion der Anströmgeschwindigkeit und des Turbulenzgrads präsentieren

Study on the Influence of Turbulence on Thermal Comfort for Draft Air

Today you can find various standards to assess thermal comfort. However, it lacks on standards for the assessment of thermal comfort in car cabins, particularly in case of high momentum air-flow or draft. Thermal comfort is often perceived differently in cars compared to e.g. in buildings or coaches. With the objective to overcome this problem, an experimental study of the impact of the draft on thermal comfort is carried out. In this paper, we present results of an investigation focusing on the influence of turbulence intensity on thermal comfort. Draft is simulated in a generic car cabin in the following parameter ranges: temperature T=17 ∘C–29 ∘C, velocity U=0.25 m/s–2.5 m/s and turbulence intensity ILT=16% and IHT=32%. The thermal comfort is determined by means of a thermal manikin and infrared thermography to evaluate the thresholds of thermal comfort based on the German Industrial Norms

Experimental study on the impact of large-scale flow structures on the passenger comfort in a miniaturised aircraft cabin

The flow structure formation and the dynamics in mixed convective air flow are studied experimentally in a miniaturised aircraft cabin equipped with thermal manikins. With the objective to obtain full-scale characteristic numbers for the down-scaled set-up, temperature measurements and particle image velocimetry are performed at high-pressure P=19.6 bar. To determine the impact of buoyancy flow on the supplied forced convective cold inflow, the flow structure formation and the heat transfer are analysed for a Grashof number range of 8.06 \times 10^9 \leq \Gr \leq 17.72 \times 10^9 and constant Reynolds number \Rey = 1.5 \times 10^5. It is found that the flow structure formation and the velocity distribution strongly depend on the ratio of buoyancy to inertia forces. In conclusion, the structure formation of the flow and its dynamics is discussed in dependency of the Richardson number \Ri considering the consequences for the thermal comfort of the passengers

Experimental study of the indoor aerosol-dynamics for a low-momentum ventilation system with an air purifier unit

We present an experimental study on the aerosol-dynamics within a test setup representing a class or conference room situation. To ensure realistic flow patterns, the blockage and heat release of the persons are simulated using 18 thermal manikins. The aerosol source is realized by a generator, attached to a thermal manikin, providing a realistic exhalation of artificial saliva. Two different ventilation scenarios are studied regarding aerosol concentration distribution and removal-efficiency. Additionally, the influence of a mask attached to the source and the effect of a moving person on the resulting aerosol concentrations are investigated. Time and spatially resolved concentrations are measured using 61 particulate matter sensors, installed on three height levels. The ventilation scenarios comprise window opening and a lowmomentum ventilation concept, where the air is extracted underneath the ceiling and reenters purified (HEPA14) on floor level. Each of the examined counter measures (open-window, low-momentum ventilation and mask) resulted in a significant lower particle concentration compared to the reference scenario. The low-momentum ventilation with an air purifier unit provided the best aerosol removal-efficiency with a decrease in concentration of up to 96%, followed by the window opening with 60%. The buoyancy flow induced by the heat loads and the resulting flow field caused by the lowmomentum ventilation concept lead to well-directed particle transport towards the ceiling. Consequently, a large amount of aerosol was extracted and filtered by the ventilation system resulting in lower particle concentrations. However, local concentrations were strongly depended on the position of the aerosol source

Transcript profiles uncover temporal and stress-induced changes of metabolic pathways in germinating sugar beet seeds

<p>Abstract</p> <p>Background</p> <p>With a cultivation area of 1.75 Mio ha and sugar yield of 16.7 Mio tons in 2006, sugar beet is a crop of great economic importance in Europe. The productivity of sugar beet is determined significantly by seed vigour and field emergence potential; however, little is known about the molecular mechanisms underlying these traits. Both traits exhibit large variations within sugar beet germplasm that have been difficult to ascribe to either environmental or genetic causes. Among potential targets for trait improvement, an enhancement of stress tolerance is considered because of the high negative influence of environmental stresses on trait parameters. Extending our knowledge of genetic and molecular determinants of sugar beet germination, stress response and adaptation mechanisms would facilitate the detection of new targets for breeding crop with an enhanced field emergence potential.</p> <p>Results</p> <p>To gain insight into the sugar beet germination we initiated an analysis of gene expression in a well emerging sugar beet hybrid showing high germination potential under various environmental conditions. A total of 2,784 ESTs representing 2,251 'unigenes' was generated from dry mature and germinating seeds. Analysis of the temporal expression of these genes during germination under non-stress conditions uncovered drastic transcriptional changes accompanying a shift from quiescent to metabolically active stages of the plant life cycle. Assay of germination under stressful conditions revealed 157 genes showing significantly different expression patterns in response to stress. As deduced from transcriptome data, stress adaptation mechanisms included an alteration in reserve mobilization pathways, an accumulation of the osmoprotectant glycine betaine, late embryogenesis abundant proteins and detoxification enzymes. The observed transcriptional changes are supposed to be regulated by ABA-dependent signal transduction pathway.</p> <p>Conclusion</p> <p>This study provides an important step toward the understanding of main events and metabolic pathways during germination in sugar beet. The reported alterations of gene expression in response to stress shed light on sugar beet stress adaptation mechanisms. Some of the identified stress-responsive genes provide a new potential source for improvement of sugar beet stress tolerance during germination and field emergence.</p

Modelling of Heat Transfer for Droplet Condensation in Mixed Convective Duct Flow

Temperature and humidity measurements are conducted in mixed convective humid-air duct flow with condensation. The latent and total heat transfer during the experiment are determined through the thermal balance for inlet temperatures from 27.5 ∘C to 35.5 ∘C, relative humidities from 30% to 55% and at four Reynolds numbers (2000–8000). The experimental results are compared with a heat transfer model from the literature. Adjusted in terms of the geometry and surface properties, the model shows partial agreement for the cases with forced convection but has to be further adjusted regarding the influence of thermal convection

Development of an experimental set-up to investigate heat transport in convective air flows with phase transition

The heat transfer in mixed convective air flows with phase transition is a phenomenon which occurs in nature and a plethora of technical applications. When condensing materials form droplets, condensation leads to an increased heat transfer rate and therefore is advantageous in heat exchangers \cite{Schmidt-1930}, whereas in other technical applications condensation is oftentimes undesirable. For instance, fogging on the windshield or headlights of motor vehicles leads to restrictions when driving and a reduction of optical transparency influences road safety. In addition, a considerable amount of thermal energy is required for defogging, which can have a negative effect on the range of electric cars \cite{WesthoffFAT}. The design and modification of these components with regard to optimizing dehumidification or preventing misting is often based on experience and intuition. Despite the enormous progress of numerical methods in recent years, there is still a lack of reliable and applicable models for the numerical simulation of condensation in general, and in particular of droplet condensation on surfaces. Due to the complexity of the physical processes that determine the heat transport and thus the fogging on the panes, reliable simulations are expensive and time-consuming, and therefore not suitable for industrial applications. Thus, the objective of the present study is to develop a model, using dimensionless numbers, which allows the prediction of the condensation behavior on panes based on a reduced parameter space. In such a configuration the mass transport of the vapor by phase transition, the resulting latent and the sensible heat transfer are determined by the physical processes of convection and diffusion, the boundary conditions and the material properties of the surfaces. In addition, the mass transfer of water vapor changes even with the smallest changes in the boundary conditions. A major challenge of this study is therefore to design and construct an experimental setup with the appropriate measurement technology, which meets the requirements of ensuring defined boundary conditions, reproducibility of the experiments and measurement accuracy. The setup corresponds to a generic replica of a vehicle headlamp

Calculation of the three-point contact angle for water droplets on a surface from a rearward droplet observation

Evaluation method to determine the three-phase contact angle from microscope images using a ray-tracing algorithm