Observations of Complex Oscillations in a Closed Thermosyphon

1985 ◽  
Vol 107 (4) ◽  
pp. 833-839 ◽  
Author(s):  
J. E. Hart
Keyword(s):  
Isothermal Section ◽  
Temperature Difference ◽  
Periodic Flow ◽  
Wide Range ◽  
Tube Assembly ◽  
Thermal Oscillations ◽  
Steady Convection

Observations have been made of thermal oscillations in a slightly inclined closed thermosyphon. The thermosyphon is made up of two isothermal tubes, capped at the outer ends, and joined along their axes by an insulating section. The tube assembly is filled with liquid and inclined slightly with respect to the vertical. The lower isothermal section is hotter than the upper one and convection is driven across the insulating region. Between the applied temperature difference at which simple steady convection occurs, and that required for persistent turbulent motions, there is a wide range over which thermal oscillations are observed. These oscillations reflect quasi-periodic flow as well as a type of periodic chaos.

IMAGING PERFORMANCE OF ADVANCED QWIP FOCAL PLANE ARRAYS

2002 ◽  
Vol 12 (03) ◽  
pp. 659-690 ◽  
Author(s):  
ARNOLD GOLDBERG
Keyword(s):  
Quantum Efficiency ◽  
Temperature Difference ◽  
Focal Plane ◽  
Focal Plane Arrays ◽  
Operating Conditions ◽  
Dual Band ◽  
Large Format ◽  
Ir Camera ◽  
Wide Range ◽  
Imaging Performance

Since the first demonstration of the quantum well infrared photodetector (QWIP) in the 1980s, there has been much progress in the application of QWIPs to the production infrared (IR) imaging systems. At this time, focal plane arrays (FPAs) made from QWIPs are readily available for insertion in IR cameras with formats as large as 640 × 480 pixels. Several organizations now have commercially available IR camera systems using QWIPs. In spite of the low single-pixel quantum efficiency relative to MCT, excellent IR imagery has been demonstrated with large format (640 × 480 pixels) single-band and moderate format (256 × 256 pixels) dual-band FPAs. With a large-format staring FPA, one can integrate the signal current for a relatively long time to produce images of similar quality to that from a scanned line array run at the same frame rate. In fact, it can be shown that due to the nature of the noise in a QWIP device, the noise performance of a QWIP FPA can be better than that of MCT FPA as long as the conversion efficiency (the product of the quantum efficiency and the photoconductive gain) is high enough for the read-out integrated circuit (ROIC) integration capacitor to be filled in a frame time. In this chapter the results of laboratory and field tests on large-format single-color QWIP FPAs operating in the LWIR band and dual-band FPAs operating in both the MWIR and LWIR bands simultaneously will be shown. Single-color and dual-band arrays will be shown to give excellent imaging performance and that dual-band FPAs offer unique capabilities to investigate the phenomenology of targets and backgrounds. The performance of the FPAs will be presented from a system performance perspective over a wide range of operating conditions (temperature, bias, integration time, etc.). Results of measurements of noise-equivalent temperature difference (NEΔT), minimum resolvable temperature difference (MRTD measured as a function of target spatial frequency), responsivity, and dark current will be reported. Imagery collected in the field will show the utility of large-format LWIR FPAs for increasing the range at which targets can be identified over previous-generation scanning imagers. Dual-band imagery collected using a QWIP FPA will show how such an array as part of a future imaging system may be able to exploit differences in the IR signatures of targets and backgrounds in the MWIR and LWIR bands to enhance the visibility of targets in cluttered environments. We also show how such an array can be used to make accurate remote temperature measurements. Finally, we will compare the performance of state-of-the-art FPAs made from QWIPs and MCT.

scholarly journals The Effect of Different Cover and Flooring Materials on Climatic Comfort in Landscape Design

2022 ◽  
Vol 9 (sp) ◽  
pp. 2571-2580
Author(s):  
Orhun Soydan ◽  
Ahmet Benliay
Keyword(s):  
Neural Network ◽  
Artificial Neural Network ◽  
Network Model ◽  
Temperature Difference ◽  
Neural Network Model ◽  
Artificial Neural Network Model ◽  
Ambient Air ◽  
Maximum Temperature ◽  
Wide Range ◽  
Artificial Neural

In this study, it is aimed to understand the effects of structural and vegetative elements that can be used in landscape designs on the temperature factor, which will greatly affect the climatic comfort, by using artificial neural networks. In this context, measurements were carried out in the morning (08:00-09:00), noon (13:00-14:00) and evening (17:00-18:00) of a total of 100 days, 50 days in each of the winter and summer seasons, at 7 randomly selected points in the Akdeniz University Campus. In these measurements, the temperature difference values of 11 cover elements on 7 different floor covering types were measured, and the ambient air temperature, humidity and wind values were also determined. The temperature differences between the areas where the flooring elements are exposed to direct sun and the shadow effect of different plant and cover elements were determined using an infrared laser thermometer. These values were processed with Neural Designer software and possible temperature difference prediction values were created for 57.750 different alternatives with the help of artificial neural network model from 837 sets of data. Evaluation shows that the maximum temperature difference is 15.6°C at noon in the summer months in the red tartan flooring material and Callistemon viminalis cover material. While the artificial neural network model predicts that there will be a high 2-3° C temperature difference for the alternatives, it has made predictions for temperature differences between 0-10°C in winter and 0-16°C in summer months. Although the temperature differences that will occur in the noon hours are distributed over a wide range of values, it seems that the morning and evening forecasts are concentrated between 0-7°C values. Also, it has been determined that the wind and humidity in the environment are more important factors than the ambient temperature in terms of temperature differences.

Acoustic oscillations driven by boundary mass exchange

2019 ◽  
Vol 866 ◽  
pp. 316-349 ◽  
Author(s):  
Avshalom Offner ◽  
Rui Yang ◽  
Daniel Felman ◽  
Nimrod Elkayam ◽  
Yehuda Agnon ◽  
...  
Keyword(s):  
Mass Transfer ◽  
Wave Equation ◽  
Limit Cycle ◽  
Latent Heat ◽  
Temperature Difference ◽  
Working Fluid ◽  
Conductive Heat ◽  
Pressure Oscillations ◽  
Wide Range ◽  
Reactive Gas

Thermoacoustic instability – self-sustained pressure oscillations triggered by temperature gradients – has become an increasingly studied topic in the context of energy conversion. Generally, the process relies on conductive heat transfer between a solid and the fluid in which the generated pressure oscillations are sustained. In the present study, the thermoacoustic theory is extended to include mass transfer; specifically, the working fluid is modified so as to incorporate a ‘reactive’ gas, able to exchange phase with a solid/liquid boundary through a sorption process (or through evaporation/condensation), such that most heat is transferred in the form of latent heat rather than through conduction. A set of differential equations is derived, accounting for phase-exchange heat and mass transfer, and de-coupled via a small-amplitude asymptotic expansion. These equations are solved and subsequently manipulated into the form of a wave equation, representing the small perturbation on the pressure field, and used to derive expressions for the time-averaged, second-order heat and mass fluxes. A stability analysis is performed on the wave equation, from which the marginal stability curve is calculated in terms of the temperature difference, $\unicode[STIX]{x0394}T_{onset}$, required for initiation of self-sustained oscillations. Calculated stability curves are compared with published experimental results, showing good agreement. Effects of gas mixture composition are studied, indicating that a lower heat capacity of the inert component, combined with a low boiling temperature and high latent heat of the reactive component substantially lower $\unicode[STIX]{x0394}T_{onset}$. Furthermore, an increase in the average mole fraction of the reactive gas, $C_{m}$ strongly affects onset conditions, leading to $\unicode[STIX]{x0394}T_{onset}\sim 5\,^{\circ }\text{C}$ at the highest value of $C_{m}$ achievable under atmospheric pressure. An analysis of the system limit cycle is performed for a wide range of parameters, indicating a systematic decrease in the temperature difference capable of sustaining the limit cycle, as well as a significant distortion of the acoustic wave form as the phase-exchange mechanism becomes dominant. These findings, combined, reveal the underlying mechanisms by which a phase-exchange engine may produce more acoustic power than its counterpart ‘classical’ thermoacoustic system, while its temperature difference is substantially lower.

scholarly journals Heat-Mass Transfer of Nanofluid in Lid-Driven Enclosure under three Convective Modes

2019 ◽  
Vol 38 ◽  
pp. 73-83
Author(s):  
MS Rahman ◽  
R Nasrin ◽  
MI Hoque
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat And Mass Transfer ◽  
Temperature Difference ◽  
Richardson Number ◽  
Numerical Study ◽  
Mass Transfer Rate ◽  
Square Enclosure ◽  
Coupled Equations ◽  
Wide Range

Heat is a form of energy which transfers between bodies which are kept under thermal interactions. When a temperature difference occurs between two bodies or a body with its surroundings, heat transfer occurs. Heat transfer occurs in three modes. Three modes of heat transfer are conduction, convection and radiation. Convection is a very important phenomenon in heat transfer applications and it occurs due to two different gradients, such as, temperature and concentration. This paper reports a numerical study on forced-mixed-natural convections within a lid-driven square enclosure, filled with a mixture of water and 2% concentrated Cu nanoparticles. It is assumed that the temperature difference driving the convection comes from the side moving walls, when both horizontal walls are kept insulated. In order to solve general coupled equations, a code based on the Galerkin's finite element method is used. To make clear the effect of using nanofluid on heat and mass transfers inside the enclosure, a wide range of the Richardson number, taken from 0.1 to 10 is studied. A fair degree of precision can be found between the present and previously published works. The phenomenon is analyzed through streamlines, isotherm and iso-concentration plots, with special attention to the Nusselt number and Sherwood number. The larger heat and mass transfer rates can be achieved with nanofluid than the base fluid for all conditions at Richardson number, Ri = 0.1 to 10. It has been found that the heat and mass transfer rate increase approximately 6% for water with the increase of Ri = 0.1 to 10, whereas these increase about 34% for nanofluid. GANIT J. Bangladesh Math. Soc.Vol. 38 (2018) 73-83

scholarly journals Effect of Vegetation Structure on Urban Climate Mitigation

2020 ◽  
Vol 23 (2) ◽  
pp. 60-65
Author(s):  
Zdenka Rózová ◽  
Ján Supuka ◽  
Ján Klein ◽  
Matej Jasenka ◽  
Attila Tóth ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Air Temperature ◽  
Temperature Difference ◽  
Urban Climate ◽  
Climate Mitigation ◽  
Wide Range ◽  
Street Space ◽  
Vegetation Stand ◽  
The City ◽  
City Park

AbstractVegetation formations are an important component in the urban structure, as they perform a wide range of ecosystem services there. The climate modification to improve the environmental and residential quality of the city is one of the important functions. The paper presents the results of the microclimate assessment in the chosen localities of Nitra town, Slovakia, with an emphasis on the stage and differences in air temperature and relative humidity. The climate elements were measured at 7 spatially different sites (sites A to G), each of them at two comparative sites, vegetation stand and open area. The largest average air temperature difference between the vegetation stand and the non-vegetation area was 1.2 °C at the locality D. The largest air temperature difference in the vegetation stands was measured between the street space (site E) and the city park (F), reaching 2.3–2.5 °C. The relative air humidity reached the highest differences between the park (locality F) and the street space (G) measured at 3:00–8:00. These reached 19.6% to 24.4% with higher relative humidity in the popular city park. The highest differences between the compared habitats were measured at locality G and averaged 9.6% at 04:00 – 07:00 in a preference to a tree canopy. The research results confirmed the importance of the vegetation structures in the process of mitigating the urban climate extremes and the environmental quality improving.

scholarly journals PTV Measurement of Periodic Flow Fields with Wide Range of Velocity

1997 ◽  
Vol 17 (Supplement2) ◽  
pp. 129-132
Author(s):  
Fujio AKAGI ◽  
Sumio YAMAGUCHI ◽  
Youichi ANDO ◽  
Yosinori KADOTA
Keyword(s):  
Flow Fields ◽  
Periodic Flow ◽  
Wide Range ◽  
Ptv Measurement

Rectal-Axillary Temperature Difference in Febrile and Afebrile Infants and Children

1993 ◽  
Vol 32 (5) ◽  
pp. 268-272 ◽  
Author(s):  
Dimitris Anagnostakis ◽  
Nicholas Matsaniotis ◽  
Stelios Grafakos ◽  
Emy Sarafidou
Keyword(s):  
Temperature Difference ◽  
Rectal Temperature ◽  
Infants And Children ◽  
Axillary Temperature ◽  
Wide Range

The rectal-axillary temperature difference (R-A) was measured in the morning, at midday, and in the afternoon on 1,519 occasions in 1,149 children from birth to 5 years old. Of these, 302 children were febrile (rectal temperature ≥38°C) and 847 were afebrile. A wide range in R-A was found for each individual in both groups. The magnitude of this difference was not associated with sex or age. In febrile children, the R-A was significantly greater ( P<.0001) at the apparent onset of fever (1.04 ± 0.25°C) than later, when fever had been present for at least two hours (0.53 ± 0.22°C). These findings indicate that it is impossible to find a standard number by which to convert axillary to rectal temperature or vice versa. Furthermore axillary temperature may be relatively low or even “normal” despite an elevated rectal temperature at the onset of fever.

Small Reynolds Number Convection in Rotating Spherical Annuli

1979 ◽  
Vol 101 (3) ◽  
pp. 427-433 ◽  
Author(s):  
R. W. Douglass ◽  
E. J. Shaughnessy ◽  
B. R. Munson
Keyword(s):  
Reynolds Number ◽  
Angular Velocity ◽  
Velocity Ratio ◽  
Perturbation Expansion ◽  
Reynolds Numbers ◽  
Regular Perturbation ◽  
Optimum Configuration ◽  
Wide Range ◽  
Boussinesq Fluid ◽  
Steady Convection

A fourth order regular perturbation expansion in powers of the Reynolds number is used to investigate the steady convection of a stratified Boussinesq fluid in rotating spherical annuli. The results include the primary and secondary flow patterns, temperature distributions, total heat flux, and torque characteristics emphasizing their dependence on a wide range of radius and angular velocity ratios. Maximum usable Reynolds numbers were found and usually were larger than 10. The optimum configuration for convective heat transfer is a radius ratio of 0.35 and an angular velocity ratio of +1/3. This same configuration gives the largest torque as well.

NUMERICAL STUDIES OF TRANSITION FROM STEADY TO UNSTEADY COUPLED THERMAL BOUNDARY LAYERS

2014 ◽  
Vol 11 (supp01) ◽  
pp. 1344002 ◽  
Author(s):  
F. XU ◽  
Q. YANG ◽  
P. YU
Keyword(s):  
Heat Transfer ◽  
Rayleigh Number ◽  
Boundary Layers ◽  
Temperature Difference ◽  
Traveling Waves ◽  
Numerical Studies ◽  
Wide Range ◽  
Rayleigh Numbers ◽  
Two Sides ◽  
Thermal Boundary Layers

The transition from the steady to the unsteady coupled thermal boundary layers around a partition in a differentially heated partitioned cavity is investigated numerically. The partitioned cavity is filled with water, which is imposed a temperature difference between the two sidewalls of the cavity. The numerical results are consistent with the corresponding experiments. The development of the unsteady coupled thermal boundary layers following a sudden temperature difference between the fluids at the two sides of the partition at a high Rayleigh number of Ra = 1011 is described. The transition from the steady to the unsteady coupled thermal boundary layers over a wide range of Rayleigh numbers from Ra = 109 to 1011 is observed. The dependence of traveling waves in the unsteady coupled thermal boundary layers on the Rayleigh number and heat transfer through the partitioned cavity are characterized and quantified.

Rotating Compressible Flow Over the Edge of a Finite Disk

1981 ◽  
Vol 48 (2) ◽  
pp. 249-254 ◽  
Author(s):  
M. Toren ◽  
A. Solan
Keyword(s):  
Compressible Flow ◽  
Temperature Difference ◽  
Incompressible Flow ◽  
Azimuthal Velocity ◽  
Ekman Layer ◽  
External Flow ◽  
Asymptotic Solutions ◽  
Rotating Flow ◽  
Outer Flow ◽  
Wide Range

Numerical and asymptotic solutions of the similarity equations governing the laminar compressible rotating flow near the edge of a finite disk are presented for a wide range of the Prandtl and Eckert numbers and the disk-to-external flow ratios of azimuthal velocity and temperature. By appropriate transformations, the compressible flow is reduced to a formulation similar to that of the incompressible flow. Wall heating and dissipation effects are shown to be equivalent to an increment of the velocity of the disk in the sense opposite to that of the outer flow. In the limit of small velocity or temperature difference between the disk and the outer flow, the solutions show how an Ekman layer is started at the edge.

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