Large Vertical Temperature Gradients Research Articles

Ceiling displacement ventilation system in an experimental theatre: A method to optimize thermal comfort

Compared to traditional theatres, experimental theatres have a lower floor height and the lights are closer to the audience. The large vertical temperature gradient caused by the underfloor air distribution (UFAD) system extremely impairs thermal comfort. Thus, an improved method by introducing the ceiling displacement ventilation (CDV) system was evaluated. CFD models of the experimental theatre were established and verified by field tests. The influence of designing and operation parameters of the CDV systems on thermal comfort was investigated. Effects on energy consumption and air quality were also analyzed to improve application feasibility. The results indicated that the increment of air temperature at the audience head (AHA temperature) was reduced by 51.0% with the help of the CDV system. Thermal comfort was improved as the installation height was extended higher. Furthermore, under constant total air supply rate, the AHA temperature of the last row was decreased with the increase in the air supply ratio of the UFAD to CDV systems (RUtoC), and the total power consumption was declined by 3.1% through adjusting RUtoC compared to the traditional UFAD system. Additionally, the air quality could be significantly improved. The results are helpful to the design and operation of HVAC systems in experimental theatres.

Large scale alignment of alumina platelets en route to porous nacre-like alumina by ice-templating

Ice templating, being a highly versatile technique contributes largely to the development of biomimetic structures. Biomimetic materials are the synthetic materials that possess one or more design and functional aspects of natural materials like abalone nacre, bone, wood. The present research aimed to replicate the inorganic part of nacre, by taking as-synthesized alpha-alumina platelets as primary building blocks. Ice templating was carried out under large vertical temperature gradient in a custom-built setup, that resulted in large-scale ordered lamellar structure; mainly due to the fast and straightforward self-assembly of platelets during directional solidification. The structural wavelength and lamella thickness, the critical parameters to define the ordered lamellar microstructure, were measured to be 16.23 ± 2.0 μm and 2.79 ± 1.0 μm respectively. The resultant green scaffold was subsequently pressed (i.e. cold-compaction) in a direction perpendicular to the ice-front growth and sintered at 1550 °C/4h. Although the nature of the compression response (before and after intermediate cold compaction step) did not change considerably, but large differences in the strength, work of fracture and modulus were obtained. This is mainly due to the large reduction of porosity as well as large-scale alignment of alumina platelets in bulk, resulted from the additional intermediate cold-compaction step. The microstructure of the sintered specimen displayed large-scale parallel alignment of anisotropic alumina platelets, closely resembles to the inorganic counterpart of natural nacre.

Исследование временной изменчивости параметров вертикальной структуры вод в прибрежной зоне южного Крыма

The issues of the formation and temporal variability of the vertical structure of the Black Sea waters off the coast of the southern Crimea are considered. A large array of instrumental measurement data obtained in the second half of 2013 (June–December) during operational monitoring of the vertical temperature profile in the area of Stationary oceanographic platform of the Marine Hydrophysical Institute in the coastal zone near Cape Kikineiz is used. The measurements were carried out using a thermoline, which is a system of temperature sensors distributed in depth from the surface to the bottom. To study the vertical structure of the temperature field, a model of two-layer stratification is considered, when two quasi-homogeneous layers (the upper one is warmer and the lower one cold) are separated by a layer of large vertical temperature gradients—the thermocline. A number of parameters of the vertical structure of waters were calculated: the vertical temperature gradient, its maximum values and depth of occurrence, the temperature of the thermocline core (if it was recorded) and quasi-uniform layers, the temperature difference between these layers, and the thermocline thickness. The median estimates of the daily values of the listed parameters are analyzed. Specific cases of transformation of the vertical profile of water temperature during the upwelling and downwelling processes are considered. It is shown that the temporal variability of the parameters of the vertical structure is significant, especially in the summer hydrological season. The main contribution to the variability is caused by the seasonal course of the heat flow from the atmosphere through the sea surface and surge processes (upwelling and downwelling) in the summer period caused by abrupt changes in the alongshore wind component. In the autumn hydrological season, the thermocline is recorded sporadically with a slight gradient in the form of the lower boundary of the subsurface layer of daytime heating, or a thin bottom layer.

Performance investigation of heating terminals in a railway depot: On-site measurement and CFD simulation

Currently, many railway sub-buildings are being newly built, where large-space workshops and depots account for more than half of the total construction area. In this study, indoor thermal environment and the effects of different heating terminals in a railway depot in the cold region of China are investigated by performing on-site measurements and computational fluid dynamics (CFD) simulations. Five heating terminals, including air-supply and radiation systems, are simulated and compared to further optimize the indoor temperature distribution. The results show that it is difficult to send hot air to the occupant zone which results in a large vertical temperature gradient and uneven indoor temperature distribution. Both radiator and fired-gas radiant tubes exhibit poor spatial uniformity, where the maximum temperature difference at the height of 1.5 m approaches 5–8 °C. The heating mode combining the radiator and radiant floor helps to achieve a better spatial temperature uniformity and a lower horizontal temperature fluctuation. The present research is beneficial for the terminal design of such types of large-space buildings.

An Efficient 1-D Thermal Stratification Model for Pool-Type Sodium-Cooled Fast Reactors

Investigating thermal stratification in the upper plenum of a sodium fast reactor (SFR) is currently a technology gap in SFR safety analysis. Understanding thermal stratification will promote safe operation of the SFR before its commercial deployment. Stratified layers of liquid sodium with a large vertical temperature gradient could be established in the upper plenum of an SFR during a down-power or a loss-of-flow transient. These stratified layers are unstable and could result in uncertainties for the core safety of an SFR. In order to predict the occurrence of the thermal stratification efficiently, we developed a one-dimensional (1-D) transport model to estimate the temperature profile of the ambient fluid in the upper plenum. This model demands much less computational effort than computational fluid dynamics (CFD) codes and provides calculations with higher fidelity than historical system-level codes. Two flow conditions were considered separately in the current study depending on if in-vessel components are presented in the upper plenum. For the condition where in-vessel components, specifically the upper internal structure, are presented, we assumed that the impinging sodium was evenly dispersed in the ambient fluid within the distance between the bottom of the in-vessel component and the jet inlet surface. For the condition where no in-vessel components are presented, we assumed that the impinging sodium was evenly dispersed in the ambient fluid within the jet length, which was determined through data-driven trainings. The newly developed 1-D model showed similar performance with the CFD model in both cases. However, due to the assumption of flat profiles of the impinging jet axial dispersion rate, nonnegligible discrepancies between the 1-D prediction and the measured data were observed.

Influence of submeso motions on scalar oscillations and surface energy balance

AbstractThe presence of wave‐like structures in the planetary boundary layer and their influence on the scalar fluxes and on the surface energy balance were investigated analyzing one year of continuous measurements collected in southern Brazil. Submeso oscillating patterns in the wind velocity components, temperature and scalar (CO, HO) concentrations were isolated using their auto‐correlation functions. The analysis showed that low wind speeds are necessary to trigger wavy motions. During night‐time, in the presence of large vertical temperature gradients, horizontal meandering and internal gravity waves are dominant features of the stable boundary layer. Furthermore, a significant number of meandering cases were identified also during daytime in neutral conditions associated with low values of net radiation. One case‐study showed how, during daytime, the wave‐like patterns may be triggered by variations in the net radiation. Spectral analysis on the whole dataset showed that oscillations in the wind velocity and temperature field are frequently associated with CO and HO wavy patterns with similar time‐scales. These non‐turbulent oscillations produce unpredictable large‐scale contributions to vertical fluxes of temperature and scalar concentrations. The energy budget analysis showed how the choice of a proper averaging time filters out these contributions and improves the energy budget closure, as well as the estimation of the net ecosystem exchange. The results confirm the influence of submeso motions in scalar dispersion, flux patterns and surface energy balance during low wind speed conditions and stable stratification.

Vertical temperature distribution characteristics and adjustment methods of a Trombe wall

This study proposed a combination of Trombe wall and a photo-thermal collector, in which, the collector was used to prepare hot water and adjust the thermal characteristics of the wall. In the hot climate of Xiangtan (China), the influence of variation in water flow of the collector on the vertical temperature gradient of Trombe wall was studied. Meanwhile, the heat transfer model of the wall system was established and verified using the experimental data. Furthermore, the distribution characteristics of vertical temperature gradient of the wall and the adjustment method were simulated using the heat transfer model. The results show that, under the action of solar irradiance, a large vertical temperature gradient occurred in the Trombe wall. The vertical temperature gradient of the massive wall and the amount of heat transfer through it could be changed by adjusting the water flow and water capacity of the collector. When the water flow was less than 100 L/(h.m2), adjusting the water flow had a significant effect on heat transfer temperature difference between the inner and outer surfaces of the wall. Compared with the polyurethane wall, the concrete wall had a smaller vertical temperature gradient, whereas its heat transfer was greater than the polyurethane wall. Additionally, increasing the thickness of the wall could effectively reduce the amount of heat transfer through polyurethane wall, however the effect on reducing the amount of heat transfer through the concrete wall was not obvious.

Mesospheric Temperature During the Extreme Midlatitude Noctilucent Cloud Event on 18/19 July 2016

AbstractA rare noctilucent cloud (NLC) event was observed at 48.8∘ N, 13.7∘ E above GERman Experimental Seismic System station in southern Germany on 18/19 July 2016 using the Compact Rayleigh Autonomous Lidar. Strong southward winds due to a quasi 2‐day planetary wave allowed for the influx of mesospheric polar air to midlatitudes on this day. The NLC observed by lidar was preceded by strong NLC displays in Cloud Imaging and Particle Size (CIPS) Experiment satellite images above the North Sea and by strong mesospheric summer radar echoes 800 km north of the lidar site and was also observed visually in central Europe. The NLC occurred at low altitude and was bright and thin with strong oscillations in altitude and brightness. Darkness allowed for high‐resolution temperature measurements at NLC altitudes. The ice particles were embedded in the upper part of a cold region with temperatures below 150 K. Significantly higher temperatures were found directly above the cloud with large vertical temperature gradients of 25 K/km at the top boundary. Spectral analysis reveals that NLC particles existed within cold phases of gravity waves within a region of high static stability. In order to study the evolution of NLC brightness in this environment, we drive the microphysical model Community Aerosol and Radiation Model for Atmospheres with lidar temperature soundings. We find that NLC particles can survive and grow in the conditions defining this midlatitude event. We conclude that the ice particles did not nucleate at the site of observation but were meridionally transported and vertically confined to a thin layer due to a large vertical temperature gradient, wind reversal, and low levels of mesospheric turbulence.

Response of tomato crop growth and development to a vertical temperature gradient in a semi-closed greenhouse

SUMMARYSemi-closed greenhouses have been developed over the last decade to conserve energy. Energy consumption is reduced by collecting the excess solar energy in Summer, storing it in aquifers and re-using it in Winter to heat the greenhouse. Cooling systems placed in the lower part of the greenhouse, can cause vertical temperature gradients, which can be more than 5°C at high levels of solar radiation. Given the substantial effect that air temperature has on a number of plant physiological processes, we expected to observe effects on plant growth and fruit production. Tomato plants were grown in semi-closed greenhouses with or without a vertical temperature gradient. The gradient was most pronounced from June to September when, for 55% of the time, the temperature difference between the top and bottom of the canopy was > 2°C, and for 20% of the time the difference was > 5°C. Despite these large vertical temperature gradients, plant growth and fruit yields were mostly unaffected. Leaf and truss initiation rates did not differ between treatments, since air temperatures at the top of the canopy were comparable. The only observed response of plants to the vertical temperature gradient was a reduced rate of fruit development in the lower part of the canopy. This resulted in a longer time between anthesis and fruit harvest in the treatment with a vertical temperature gradient, and an increase in the average fruit weight in Summer. However, total fruit production over the whole season was not affected. These results are important when designing greenhouses, as well as heating and cooling systems for greenhouses.

Substantiation of the possibility of direct measurements of radiative heat influx in the atmosphere

Numerical experiments simulating measurements with ideal (linear) and real receivers are performed to estimate the possibility of using an optical-acoustic detector for measuring a radiative heat influx (RHI) based on line-by-line calculations. The error of the real receiver is estimated with respect to an ideal one. To this end, a long-wave RHI is calculated in detectors of different sizes where the concentrations of water vapor and carbonic acid gas and the spectral composition of incident radiation is characteristic for the atmospheric surface layer. The consideration of some hypothetical situations allows us to simulate a wide range of conditions in the atmospheric surface layer and to evaluate how they affect measurement errors. Line-by-line calculations of absorption are performed using the HITRAN database. A connection between the relative error attributed to the nonlinearity of absorption, the size of the receiver, and the measurement conditions is analyzed. This is vitally important in the case when large vertical temperature gradients are observed at the point of measurements. For a receiver not larger than 2 cm and in case of normal conditions in the atmospheric surface layer, the error does not exceed 10–20%. The calculation confirms the admissibility of using optical-acoustic detectors for direct measurements of the radiative heat influx in the atmosphere.

Diagnostics of the Jovian Aurora Deduced from Ultraviolet Spectroscopy: Model and HST/GHRS Observations

A model coupling an electron energy degradation code with a detailed synthetic spectrum of the H 2 Lyman and Werner band system is used to calculate the emerging auroral ultraviolet spectra from Jupiter's atmosphere excited by electrons with different initial energy distributions. The atmospheric model is adapted from the vertical P-T profile measured by the Galileo probe and mid-latitude model hydrocarbon photochemistry. Each altitude layer, with its own gas temperature, contributes to the emergent ultraviolet spectrum and the absorbers are vertically distributed within the source region of the auroral emissions. Examples of the calculated spectra are shown to validate the synthetic spectrum and to illustrate the importance of the electron energy distribution and the vertical structure. The model is then applied to the analysis of seven HST/GHRS spectra of the 1200–1700 Å region obtained with 5-Å resolution at various locations in the north and south Jovian aurora. These spectra have different color ratios which characterize the energy of the precipitated electrons, although they do not have a high enough spectral resolution to permit a determination of the H 2 temperature. We find that the characteristic energy of the assumed initial Maxwellian distribution ranges between 17 and 40 keV. A clear signature of acetylene absorption is observed near 1520, 1480, and 1440 Å where the C 2H 2 cross section shows strong absorption peaks. The acetylene column abundance overlying the emission peak varies from 0.02 to 0.2 of the methane column. A better fit is obtained for some spectra when ethane absorption is added. The C 2H 6 column abundance varies from 0 to 0.5 of the methane column. These changes relative to methane are presumably the result of perturbations by heat released by the fast electron thermalization and/or perturbations to the hydrocarbon chemistry resulting from the production of H atoms by the aurora. A spectrum of the Io flux tube footprint and its trailing tail shows an ultraviolet color and hydrocarbon absorption quite similar to some of the main oval spectra. This observation suggests that the electrons of the Io flux tube are energized to a few tens of keV, similar to the electron precipitated in the main ovals and polar caps. Echelle spectra between 1216 and 1220 Å at 0.07 Å resolution are also compared with the model fitting best the mid-resolution spectra. It is found that the effective H 2 rovibrational temperature associated with the echelle spectra are significantly higher than predicted by the mid-latitude model. A large vertical temperature gradient just above the methane homopause due to large heating by auroral precipitation is a plausible explanation for this difference.

Natural convection of high Prandtl number fluids with variable viscosity in a vertical slot

Natural convection of high Prandtl number fluids with temperature-dependent viscosity in a vertical slot with aspect ratio 15 is studied numerically. A four-parameter fit for the variable viscosity is adopted. The flow patterns and the critical Grashof numbers predicted are in good agreement with earlier experimental results of Chen and Thangam [ J. Fluid Mech., 161, 161–173 (1985)]. The critical Grashof number is lower than the corresponding value obtained for the constant viscosity fluid, and it varies almost inversely with the Prandtl number. The viscosity stratification and the large vertical temperature gradient in the lower region of the slot cause the secondary cells to appear in the upper region of the central core at onset. The heat transfer rate is slightly smaller than that in constant property case as the viscosity variation becomes large.

Vertical Profiles of Temperature and Ozone Observed during DYANA Campaign with the NIES Ozone Lidar System at Tsukuba.

Temperature and ozone profiles were observed with the NIES ozone lidar system during the DYANA campaign from January 15th to March 15th at Tsukuba (36°E, 140°N). Temperature profiles from 30km to 90km and ozone profiles from 20km to 45km were obtained. The mesospheric temperature profiles were highly variable and deviations from the NASA88 model atmosphere were large in January and February, but the deviations were small in March 1990. Especially, conspicuous variations of the mesospheric temperature profiles, rapid increase around 55km and rapid decrease around 75km, were observed during the period from January 24th to 26th. We also observed layers with large vertical temperature gradients close to the adiabatic lapse rate above clear inversion layers in the middle mesosphere on January 17th and February 17th. The observed ozone number densities were>10% lower than the ozone sonde data averaged over 22 years at 25km and 30km on January 17th, 25th, 26th. The deviations from the averaged ozone sonde data were small on February 5th, March 8th, 10th and 13th.

Reply to Prof. Lettau

of the soil/air interface, which is a parameter theoretically convenient for evaluating G. Similar differences of views are not rare in the scientific literature. Sutton (1953, p. 206), for example, states in this respect: “the data may be analyzed in two ways, (1) by following the lead of a theory which has undergone sufficient mathematical development to make possible detailed comparison with measurement or (2) by investigation of the empirical relations which may serve subsequently as the foundation of a rational theory”. At times, however, the boundary between these two approaches is difficult to define. We thank Prof. Lettau for his remarks and for giving us the opportunity to explain the three reasons for our proposal. The first reason is that the rigorously derived Lettau formulae are theoretically satisfying but they are expressed as a function of the temperature at the soil/air interface. However, “the surface temperatures of leaves or the ground are difficult to measure. Very fine-wire electrical thermometers can be attached to leaves or appressed to the ground, but even using a large number may not give an adequate spatial sample. Probably the best approach is to sense the surface temperature remotly by a radiation thermometer” (Oke, 198 1, pp. 310-3 11). Oke also discussed the limits of radiometric measurements (e.g. error due to emissivity: about 1 “C). These difficulties cannot be avoided by extrapolating air or soil profiles, as mentioned by Munn (1966, p. 24): “the large vertical temperature gradients that exist near the surface are illustrated; the impossibility of extrapolating profiles to obtain a surface temperature is evident”. The second reason is philosophical in nature and is herewith expressed by Prof. Lettau (Lettau, 1979, p. 451): “the primary external factor is solar flux density and its absorption and/or the resulting net radiation at the active surface”. “The net radiation is the main determinant of the climate. The magnitude of the net radiation of the underlying surface greatly affects the distribution of temperature in the soil and adjacent air layers, which accounts for the particular role of the net radiation in the calculation of evaporation and snow melting, and also in a number of problems concerned with weather forecasting” (Kondratiev, 1969, p. 657). For this reason, we searched for an equation to calculate G, H, and

On rotating thermal convection driven by non-uniform heating from below

Experiments are described in which a radial temperature gradient is maintained along the lower horizontal boundary of a rotating annulus containing a thermally convecting fluid; the vertical side walls and upper horizontal boundary are nominally insulating. Comparison is made with the non-rotating experiments of Rossby (1965) and the same general asymmetric circulation is observed, i.e. that of a weakly stratified interior of slowly descending fluid occupying most of the annular gap, overlying a thin thermal layer of large vertical temperature gradients, stable over the cold part of the base and statically unstable over the warmer part; the circulation is completed by a narrow region of rising motion at the warm end of the base.A boundary-layer scaling analysis demonstrates the existence of six flow regimes, depending on the magnitude of a quantity Q defined such that Q is the square of the ratio of the (non-rotating) thermal-layer scale to the Ekman-layer scale. For small Q the flow is only weakly modified by rotation but as Q increases past unity rotation tends to thicken the thermal layer. Also presented are some numerical similarity solutions for the special case of a quadratic temperature distribution on the lower boundary and partially covering the range of Q achieved in the experiments, which is zero to ten. Above a certain critical value of Q (for the geometry used here Qc = 3·4) a baroclinic wave regime exists but is not examined in detail here although a brief discussion of an instability problem is given. Throughout comparisons are drawn between the experimental results and theoretical aspects of the problem.It is thought that the essential features of a system thermally driven in this way have their counterparts in natural systems such as the large-scale thermally induced ocean circulation driven by the latitudinal variation of incoming solar radiation.