Steady Natural Convection from a Vertical Hot Plate with Variable Radiation

The natural convection from a vertical hot plate with radiation and constant flux is studied numerically to know the velocity and temperature distribution characteristics over a vertical hot plate.The governing equations of the natural convection in two-dimension are solved with the implicit finite difference method,whereas the discretized equations are solved with the iterative relaxation method.The results show that the velocity and the temperature increase along the vertical wall.The influence of the radiation parameter in the boundary layer is significant in increasing the velocity and temperature profiles.The velocity profiles increase with the increase of the radiation parameter.The temperature profiles near the wall plate parallel each other due to the constant heat flux applied to the wall.The influence of the radiation parameter is significant either in velocity or temperature characteristics.At the same time,the effect of the Prandtl number greater than 0.71 is not sensitive to the velocity and temperature variations elsewhere.

Numerical simulation on heat transfer performance of vertical U-tube with different borehole fill materials

Heat exchange performance of vertical U-tube heat exchanger was studied with two different borehole fill materials and CFD software. Borehole surface temperature and water temperature distribution were simulated on the condition of continuous operation for 8 h in winter with inlet water temperature being 10℃. The results show that there is no obvious difference on heat exchanger performance between the two different borehole fill materials.

Oceanic vertical mixing of the lower halocline water in the Chukchi Borderland and Mendeleyev Ridge

Oceanic vertical mixing of the lower halocline water(LHW)in the Chukchi Borderland and Mendeleyev Ridge was studied based on in situ hydrographic and turbulent observations.The depth-averaged turbulent dissipation rate of LHW demonstrates a clear topographic dependence,with a mean value of 1.2×10^(-9) W/kg in the southwest of Canada Basin,1.5×10^(-9) W/kg in the Mendeleyev Abyssal Plain,2.4×10^(-9) W/kg on the Mendeleyev Ridge,and2.7×10^(-9) W/kg on the Chukchi Cap.Correspondingly,the mean depth-averaged vertical heat flux of the LHW is0.21 W/m^(2) in the southwest Canada Basin,0.30 W/m^(2) in the Mendeleyev Abyssal Plain,0.39 W/m^(2) on the Mendeleyev Ridge,and 0.46 W/m^(2) on the Chukchi Cap.However,in the presence of Pacific Winter Water,the upward heat released from Atlantic Water through the lower halocline can hardly contribute to the surface ocean.Further,the underlying mechanisms of diapycnal mixing in LHW—double diffusion and shear instability—was investigated.The mixing in LHW where double diffusion were observed is always relatively weaker,with corresponding dissipation rate ranging from 1.01×10^(-9) W/kg to 1.57×10^(-9) W/kg.The results also show a strong correlation between the depth-average dissipation rate and strain variance in the LHW,which indicates a close physical linkage between the turbulent mixing and internal wave activities.In addition,both surface wind forcing and semidiurnal tides significantly contribute to the turbulent mixing in the LHW.

On the variability of vertical eddy heat flux in the upper ocean

Ocean eddies produce strong vertical heat flux(VHF)in the upper ocean,exerting profound influences on the climate and ecosystem.Currently,mooring array provides a standard way to estimate the eddy-induced VHF(EVHF)based on the adiabatic potential density equation.Apart from the validity of adiabatic assumption,it remains unclear to what extent the estimated EVHF at a single location within a limited time period is representative of its climatological mean value.In this study,we analyzed the above issue by systematically evaluating the variability of EVHF simulated by a 1-km ocean model configured over the Kuroshio Extension.It is found that the EVHF at a single location exhibits pronounced variability.Even averaged over one year that is comparable to the current maintenance capacity of mooring array,the EVHF still deviates significantly from its climatological mean value.For more than 49%of locations in our computational domain(31°-40°N,149°-166°E),the discrepancy between the one-year mean EVHF and its climatological mean value at the peaking depth is larger than the climatological mean itself.The mesoscale eddies play a dominant role in the variability of EVHF but contribute little to the climatological mean EVHF;the opposite is true for submesoscale eddies.Our findings indicate that nested mooring array allowing for isolating the effects of submesoscale eddies will be useful to obtain climatological mean EVHF.

Spatial Distributions of Atmospheric Radiative Fluxes and Heating Rates over China during Summer

The latitude-altitude distributions of radiative fluxes and heating rates are investigated by utilizing CloudSat satellite data over China during summer. The Tibetan Plateau causes the downward shortwave fluxes of the lower atmosphere over central China to be smaller than the fluxes over southern and northern China by generating more clouds. The existence of a larger quantity of clouds over central China reflects a greater amount of solar radiation back into space. The vertical gradients of upward shortwave radiative fluxes in the atmosphere below 8 km are greater than those above 8 km. The latitudinal-altitude distributions of downward longwave radiative fluxes show a slantwise decreasing trend from low latitudes to high latitudes that gradually weaken in the downward direction. The upward longwave radiative fluxes also weaken in the upward direction but with larger gradients. The maximum heating rates by solar radiation and cooling rates by longwave infrared radiation are located over 28 40°N at 7 8 km mean sea level (MSL), and they are larger than the rates in the northern and southern regions. The heating and cooling rates match well both vertically and geographically.

ON A POSSIBLE MECHANISM FOR SOUTHERN ASIAN CONVECTION INFLUENCING THE SOUTH ASIAN HIGH ESTABLISHMENT DURING WINTER TO SUMMER TRANSITION

The establishment of the South-Asian high (SAH) in April and May over the Indochina Peninsula (IP) is investigated based on the ERA-40 reanalysis data. The result shows that the SAH is generated and strengthened over the IP locally, rather than moving westward to the IP from the Western Pacific. After the SAH establishment the tropical upper tropospheric trough (TUTT) forms above the ocean to the east of the Philippines. We have found that the principal triggering factor of both the SAH construction and the TUTT formation is the variation in the Southern Asian atmospheric diabatic heating regime. In late April, both the climbing effect of Shan Plateau and the local surface sensible heating contribute to local rainfall over the IP. Then the local updraft and upper-air divergence are strengthened, being responsible for the SAH formed in the southern part of the IP. As convection moves northward along the Australian-Asian "maritime continent" and the Bay of Bengal (BoB) summer monsoon begins, the convection is intensified in May on the eastern BoB. The strong convection results in the SAH enhancing and expanding westward, accompanied by reinforced meridional flow to the east of SAH, where responses of the circulation to diabatic heating arrive at a quasi-steady state. Meanwhile, because of the positive geopotential vorticity advection resulting from upper equatorward flow, the local positive relative vorticity increases over the ocean to the east of the Philippines, making the tropical upper tropospheric trough (TUTT) form around 150°E.

Fabrication of high-quality three-dimensional photonic crystal heterostructures

Three-dimensional photonic crystal （PC） heterostructures with high quality are fabricated by using a pressure controlled isothermal heating vertical deposition technique. The formed heterostructures have higher quality, such as deeper band gaps and sharper band edges, than the heterostructures reported so far. Such a significant improvement in quality is due to the introduction of a thin TiO2 buffer layer between the two constitutional PCs. It is revealed that the disorder caused by lattice mismatch is successfully removed if the buffer layer is used once. As a result, the formed heterostructures possess the main features in the band gap of constitutional PCs. The crucial role of the thin buffer layer is also verified by numerical simulations based on the finite-difference time-domain technique.

Effect of seasonal barrier layer on mixed-layer heat budget in the Bay of Bengal

Time series measurements (2010–2017) from the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) moorings at 15°N,90°E and 12°N,90°E are used to investigate the effect of the seasonal barrier layer (BL) on the mixed-layer heat budget in the Bay of Bengal (BoB).The mixed-layer temperature tendency (?T/?t) is primarily controlled by the net surface heat flux that remains in the mixed layer(Q’) from March to October,while both Q’and the vertical heat flux at the base of the mixed layer (Q_(h)),estimated as the residual of the mixed-layer heat budget,dominate during winter (November–February).An inverse relation is observed between the BL thickness and the mixed-layer temperature (MLT).Based on the estimations at the moorings,it is suggested that when the BL thickness is≥25 m,it exerts a considerable influence on ?T/?t through the modulation of Q_(h) (warming) in the BoB.The cooling associated with Q_(h) is strongest when the BL thickness is≤10 m with the MLT exceeding 29°C,while the contribution from Q_(h) remains nearly zero when the BL thickness varies between 10 m and 25 m.Temperature inversion is evident in the BoB during winter when the BL thickness remains≥25 m with an average MLT<28.5°C.Furthermore,Q_(h) follows the seasonal cycle of the BL at these RAMA mooring locations,with r>0.72 at the 95%significance level.

What Determines the Amplitude of ENSO Events?

In this paper, the dynamic effect of oceanic upwelling on the intensity of E1 Nifio-Southern Oscilla tion （ENSO） is studied using a simple coupled model （Zebiak-Cane Model）. The term balance analysis in the temperature variability equation shows that the anomalous upwelling of the mean vertical temperature gradient and the mean advection of the anomalous meridional tem perature gradient are the two of most important factors that determine the intensity of ENSO events, in which the ＂vertical oceanic heat flux＂ in the eastern equatorial Pa cific （EEP） is the primary influencing factor. The lag cor relation between ＂vertical heat flux （VHF）＂ and ENSO intensity shows that the highest correlation occurs when the former leads the latter by one to two weeks. The VHF is positively correlated with the background thermocline strength in the EEP, and an increase of both could result in strong ENSO variability. Comparison of the forced and coupled experiments suggests that the coupled process can affect both the intensity and freauencv of ENSO.

Advances in ground heat exchangers for space heating and cooling:Review and perspectives

As a renewable energy source,geothermal energy has been widely used to provide space heating and cooling for buildings.The thermal performance of ground heat exchanger(GHE)is significant for the operating efficiency of the ground source heat pump(GSHP)systems.This paper presents a comprehensive review of developments and advances of three kinds of GHE,including vertical borehole GHE(VBGHE),Pile GHE(PGHE),and deep borehole GHE(DBGHE)which are currently popular in larger GSHP systems.Firstly,analytical models proposed to ana-lyze heat transfer process of VBGHE with different geological conditions are summarized,such as homogenous or heterogeneous ground,with or without groundwater advection.Numerical and short-time step models and measures to improve GHE thermal performance are also reviewed.Secondly,a summary of research advances in PGHE is provided,which includes the heat transfer models of PGHE,the effects of geometric structure,oper-ation modes,pile spacing,use of phase change material(PCM),thermal properties of PCM,thermo-mechanical behavior and/or thermal performance of PGHE.The effects of groundwater flow direction and velocity on PGHE are also summarized in brief.Lastly,models of three kinds of DBGHEs,i.e.,deep coaxial GHE(DCGHE),deep U-bend GHE(DUGHE)and super-long gravity heat pipe(SLGHP),are reviewed.The physical bases of the dif-ferent analytical models are elaborated and also their advantages and disadvantages are described.Advances in numerical modelling and improving numerical model calculation speed of DCBHE,DCBHE array,and DUBHE are summarized.The review provides a meaningful reference for the further study of GHEs.

Numerical Simulation on Heat Transfer Enhancement of Phase Change Thermal Storage Devices for Low-middle Temperature

Using Ba（OH）2·8H2O as phase change material（PCM） and water as heat transfer fluid（HTF）,we numerically simulated annular finned-tube heat exchangers.In order to measure and analyze the impact of parameters in the heating/cooling process,temperature changes of different monitoring points,fin widths,and fin pitches as key parameters were considered and applied.The experimental results show that the heat exchange process can be divided into three stages within a certain time.The faster heat transfer rate is associated with the greater temperature difference between PCM and HTF.Furthermore,fins width and pitch affect dramatically the heat charging/discharging rate.The large fins width or small fins pitch is beneficial for extending the heat exchange surface,leading to improve heat transfer efficiency.

THE SIMULATION ANALYSIS OF A PROPOSED‘HYDRONIC RADIATOR’SYSTEM TOWARDS LOW COST HOUSING OPERATION IN TEMPERATE AND HOT TROPICAL CLIMATES

Fuel poverty is one of the global concerns affecting not only users’financial capacity or affordability for maintaining housing operation but also the occupants’health and wellbeing.Space heating and cooling require a relatively large amount of domestic energy use in housing.Therefore,this study was formed with the aim to propose an innovative approach to utilising free,clean renewable sources of energy applicable to the space heating and cooling of housing in both cold and hot regions.Accordingly,housing test facilities based in Melbourne,Australia,and Kuching,Malaysia,were selected and used for this study that examined the thermal performance of a proposed‘hydronic radiator’(HR)system through simulation and onsite measurements.The geothermal heat capacity of a‘vertical ground heat exchanger’(VGHE)installed in the house in Melbourne was examined previously by the authors and the VGHE measured data was also applied to this HR performance simulation.The water that circulates through the HRs is heated by sunlight and VGHE or cooled by night sky radiation.This study drew conclusions that the sole utilisation of renewable sources through these proposed HR space heating and cooling systems can provide thermally accessible or comfortable indoor living environments in both heating or cooling dominant regions.Thus,fuel poverty issues may be alleviated through HR system application.The HRs can remove a‘sensible’portion of metabolic heat,but they cannot effectively contribute to the‘latent’heat removal.Thus,the future potential use or effect of‘flow-through’HRs,which are integrated into a underfloor air distribution(UFAD)plenum,was also dsicussed in this study.In the test house located in Melbourne,the flow-through HR UFAD system is currently under development.Therefore,the performance will be measured once the system has come into operation for further testing.

Modification of the land surface energy balance relationship by introducing vertical sensible heat advection and soil heat storage over the Loess Plateau

Little is known about the surface energy balance problem for a complex underlying surface.Taking data from the Loess Plateau Land-surface Processes Experiment(LOPEX) and investigating the characteristics of the surface energy balance over a complex underlying surface,this paper calculates the soil heat storage and vertical sensible heat advection,analyzes their contributions to the surface energy imbalance,and discusses the mechanism by which the vertical velocity and temperature gradient in the surface layer affect the vertical sensible heat advection transfer.We found that the vertical velocity in the surface layer provides the necessary dynamic power for vertical sensible heat advection,and a relatively strong temperature gradient is the energy source generating vertical sensible heat advection.Under an ascending condition,the effect of vertical sensible heat advection on the surface energy budget is more obvious.It is also found that when the soil heat storage term and the vertical sensible heat advection term are added to the energy balance equation,the imbalance significantly improves.The peak of average diurnal residuals decreases from 125.1 to 41.5 W m-2,the daily average absolute value of residuals falls from 59.0 to 26.4 W m-2,and the surface energy balance closure increases from 78.4% to 94.0%.

Numerical simulation for thermal response test performance in closed-loop vertical ground heat exchanger

In this study, a series of numerical analyses was performed in order to evaluate the performance of full-scale closed-loop vertical ground heat exchangers constructed in Wonju, South Korea. The circulating HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the ground heat exchanger system. Two user-defined functions (UDFs) accounting for the difference in the temperature of the circulating inflow and outflow fluid and the variation of ground temperature with depth were adopted in the FLUENT modeling. The thermal conductivities of grouts (cement vs. bentonite) measured in laboratory were used as input values in the numerical analyses to compare the thermal efficiency of the cement and bentonite grouts used for installing the closed-loop vertical ground heat exchanger. A series of numerical analyses was carried out to simulate in-situ thermal response tests performed in the construction site. From the comparison between the in-situ thermal response test results and numerical simulations, the average thermal conductivity of the ground formation in the construction site is back-calculated as approximately 4 W/mK. This value can be used in evaluating the long-term performance of the closed-loop vertical ground heat ex changer.

Flow and Heat Transfer Instability of Supercritical Carbon Dioxide in a Vertical Heated Tube

Supercritical carbon dioxide(S-CO_(2))is one of the most promising working fluids in energy conversion systems.However,the instability of the flow and heat transfer has caused great harm to the security of energy conversion systems.In this work,a transient model based on the Finite Volume Method is set up to investigate the flow and heat transfer instability of CO_(2) changing from a subcritical state to a supercritical state in a vertical heated circular tube.The instability occurs when the wall heat flux is higher than a critical value,which makes the density and mass flow rate variations large enough.A large variation of the density triggers self-sustained oscillations in the flow.The critical heat flux heightens with the higher inlet pressure and pressure drop,larger tube diameter,and lower inlet temperature,but it reduces with the lengthening of the tube.To reflect the density-variation degree for the corresponding heat flux,a dimensionless number N_(tpc)=qπDL/Mβ_(pc)(p)/C_(p,pc)(p)(trans-pseudocritical number)is introduced.The critical trans-pseudocritical number Ntpc,c first goes up and down with the increase of the inlet pressure and the reduction of the inlet temperature.The rise of the mass flow rate,the shortening of the tube length,and the enlargement of the tube diameter all induce the temperature difference along the radial direction to become large.These tendencies make the critical Ntpc,c small.Consequently,the stability boundary N_(tpc,c)=48.47(N_(psc))^(1.048)(Δp∗)^(0.359)(D/L)^(−0.026)(G∗)^(−0.335)(ρ∗)^(2.666) is obtained to distinguish the regions of the flow and heat transfer stability and instability.

Experimental evaluation of a“U”type earth-to-air heat exchanger planned for narrow installation space in warm climatic conditions

The thermal performance of a“U”type earth-to-air heat exchanger is presented in this experimental study.The device has a serial-connected vertical configuration.The wells where tubes were installed have a depth of fewer than 3 m and are separated every 1.5 m,using an installation area of 3m2.The experimentation was carried out in March in Morelos,Mexico when the environmental temperature reaches 35℃ during the day.The performance of the device was measured and compared to the requirements of an office for cooling purposes within a university campus to reproduce the space restrictions found in urbanized areas.By using a small land surface,it is feasible for urbanized areas.The air temperature inside the“U”type earth-to-air heat exchanger,the surrounding soil temperature,the airspeed,and the power consumed by the fan were measured.The air temperature and the fan’s power consumption data were obtained by modifying the airspeed in four constant values,from 1.3 m/s to 6.6 m/s.Results show that the device evaluated in this work has adequate thermal performance for cooling purposes compared to the requirements of an office.A decrease in air temperature was recorded in a range of 5.1℃ to 9.4℃.Over 70%of the total temperature difference was reached in the first well,where the average soil thermal disturbance at 5 cm was 2.8℃.The device achieved a maximum COP of 12.8 and a maximum effectiveness of 88.4%.With these results,it is concluded that the system is suitable for cooling purposes in areas with space restrictions.This work is novel since the dimensions available for installation in urbanized areas are considered and compared with the thermal requirements of an office.In addition to the fact that there are no published works with vertical heat exchangers connected in series.