Scavenging of Soluble Gaseous Pollutants by Rain Droplets in the Atmosphere With Nocturnal Temperature Profile

We analyze non-isothermal absorption of trace gases by the rain droplets with internal circulation which is caused by interfacial shear stresses. It is assumed that the temperature and concentration of soluble trace gases in the atmosphere varies in a vertical direction. The rate of scavenging of soluble trace gases by falling rain droplets is determined by solving heat and mass transfer equations. In the analysis we accounted for the accumulation of the absorbate in the bulk of the falling rain droplet. The problem is solved in the approximation of a thin concentration and temperature boundary layers in the droplet and in the surrounding air. We assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and concentration of the absorbate and temperature are homogeneous and time-dependent in the bulk. By combining the generalized similarity transformation method with Duhamel’s theorem, the system of transient conjugate equations of convective diffusion and energy conservation for absorbate transport in liquid and gaseous phases with time-dependent boundary conditions is reduced to a system of linear convolution Volterra integral equations of the second kind which is solved numerically. Calculations are performed using available experimental data on nocturnal temperature profiles in the atmosphere. It is shown than if concentration of a trace gas in the atmosphere is homogeneous and temperature in the atmosphere increases with altitude, droplet absorbs gas during all the period of its fall. Neglecting temperature inhomogenity in the atmosphere described by nocturnal temperature inversion leads to essential underestimation of the trace gas concentration in a droplet on the ground.

From Megawatt to Gigawatt: New Developments in Concentrating Solar Thermal Power

On October 30th 2009, a major industrial consortium initiated the so-called DESERTEC project which aims at providing by 2050 15% of the European electricity from renewable energy sources in North Africa, while at the same time securing energy, water, income and employment for this region. In the heart of this concept are solar thermal power plants which can provide affordable, reliable and dispatchable electricity. While this technology has been known for about 100 years, new developments and market introduction programs have recently triggered world-wide activities leading to the present project pipeline of 8.5 GW and 42 billion Euro. To become competitive with mid-load electricity from conventional power plants within the next 10–15 years, mass production of components, increased plant size and planning/operating experience will be accompanied by technological innovations which are presently in the development or even demonstration stage. The scale of construction, the high temperatures and the naturally transient operation provide formidable challenges for academic and industrial R&D. Experimental and theoretical research involving all mechanisms of heat transfer and fluid flow is required together with large-scale demonstration to resolve the combined challenges of performance and cost.

Influence of Force Fields and Flow Patterns on Boiling Heat Transfer Performance

Recent experimentation of boiling in different environments, namely in reduced or enhanced gravity and/or in the presence of electric fields, have shed new light on the comprehension of boiling phenomena and have focused the objectives of future investigation. The recent results achieved by the author and other research groups around the world are reported and discussed in the paper. After a short introduction on some fundamental phenomena and their dependence on force fields, pool and flow boiling are dealt with. In particular, it is stressed that due to increased coalescence peculiar flow regimes take place in reduced gravity, influencing the heat transfer performance. The application of an electric field may, in some instances, delay or avoid these regime transitions. In boiling at high flowrate, the phenomena are dominated by inertia and thus gravity-independent; however the threshold at which this occurs has still to be determined.

Interfacial Thermal Fluid Phenomena in Thin Liquid Films

Films are ubiquitous in nature and play an important role in our daily life. The paper focuses on the recent progress that has been achieved in the interfacial thermal fluid phenomena in thin liquid films and rivulets through conducting experiments and theory. Phase shift schlieren technique, fluorescence method and infrared thermography have been used. A spanwise regular structures formation was discovered for films falling down an inclined plate with a built-in local rectangular heater. If the heating is low enough, a stable 2D flow with a bump at the front edge of the heater is observed. For lager heat flux this primary flow becomes unstable, and the instability leads to another steady 3D flow, which looks like a regular structure with a periodically bent leading bump and an array of longitudinal rolls or rivulets descending from it downstream. The heat flux needed for the onset of instability grows almost linearly with the increase of Re number. Strong surface temperature gradients up to 10–15 K/mm, both in the streamwise and spanwise directions have been measured. For a wavy film it was found that heating may increase the wave amplitude because thermocapillary forces are directed from the valley to the crest of the wave. Thin and very thin (less than 10 μm) liquid films driven by a forced gas/vapor flow (stratified or annular flows), i.e. shear-driven liquid films in a narrow channel are a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Development of such technology requires significant advances in fundamental research, since the stability of joint flow of locally heated liquid film and gas is a rather complex problem. Experiments with water and FC-72 in flat channels (height 0.2–2 mm) have been conducted. Maps of flow regimes were plotted. It was found that stratified flow exists and stable in the channels with 0.2 mm height and 40 mm width. The critical heat flux for a shear driven film may be up to 10 times higher than that for a falling liquid film, and reaches 400 W/cm2 in experiments with water at atmospheric pressure. Some experiments have been done during parabolic flight campaigns of the European Space Agency under microgravity conditions. It was found that decreasing of gravity leads to a flow destabilization.

Numerical Simulation for Outdoor Thermal Environment of High-Rise Building With Air-Conditioner

A large number of split-type air conditioner are widely used in high-rise residential or office buildings in China and the outdoor units of air-conditioner are often installed at the sidewalls or on the roofs in the confined space of a high-rise building. The factors affecting the performance of air-conditioner are the solar radiation, the heat released from the outdoor units, the ventilation of the confined installation space of a building where the outdoor units are installed and so on, which are investigated in this study. The air flow and temperature distribution under steady-state condition near the two outdoor units installed on the same storey in a building are simulated by software FLUENT, in which the porous model and DO radiation model are used. The optimum installation distance from the supporting wall is obtained. The average temperature of the exit surface without wind is 1.18% more than that with wind. The results show that the heat released from the outdoor units and the ventilation of the confined installation space where the outdoor units are installed are the main factors affecting the thermal environment in the confined installation space; The influence of the solar radiation can be neglected.

Reynolds, Maxwell, and the Radiometer, Revisited

In 1969, S. G. Brush and C. W. F. Everitt published a historical review, that was reprinted as subchapter 5.5 Maxwell, Osborne Reynolds, and the radiometer, in Stephen G. Brush’s famous book The Kind of Motion We Call Heat. This review covers the history of the explanation of the forces acting on the vanes of Crookes radiometer up to the end of the 19th century. The forces moving the vanes in Crookes radiometer (which are not due to radiation pressure, as initially believed by Crookes and Maxwell) have been recognized as thermal effects of the remaining gas by Reynolds — from his experimental and theoretical work on Thermal Transpiration and Impulsion, in 1879 — and by the development of the differential equations describing Thermal Creeping Flow, induced by tangential stresses due to a temperature gradient on a solid surface by Maxwell, earlier in the same year, 1879. These fundamental physical laws have not yet made their way into the majority of textbooks of heat transfer and fluid mechanics so far. A literature research about the terms of Thermal Transpiration and Thermal Creeping Flow, in connection with the radiometer forces, resulted in a large number of interesting papers; not only the original ones as mentioned in subchapter 5.5 of Brush’s book, but many more in the earlier twentieth century, by Martin Knudsen, Wilhelm Westphal, Albert Einstein, Theodor Sexl, Paul Epstein and others. The forces as calculated from free molecular flow (by Knudsen), increase linearly with pressure, while the forces from Maxwell’s Thermal Creeping Flow decrease with pressure. In an intermediate range of pressures, depending on the characteristic geometrical dimensions of flow channels or radiometer vanes, an appropriate interpolation between these two kinds of forces, as suggested by Wilhelm Westphal and later by G. Hettner, goes through a maximum. Albert Einstein’s approximate solution of the problem happens to give the order of magnitude of the forces in the maximum range. A comprehensive formula and a graph of the these forces versus pressure combines all the relevant theories by Knudsen (1910), Einstein (1924), Maxwell (1879) (and Hettner (1926), Sexl (1928), and Epstein (1929) who found mathematical solutions for Maxwells creeping flow equations for non-isothermal spheres and circular discs, which are important for thermophoresis and for the radiometer). The mechanism of Thermal Creeping Flow will become of increasing interest in micro- and submicro-channels in various new applications, so it ought to be known to every graduate student of heat transfer in the future. That’s one of the reasons why some authors have recently questioned the validity of the classical Navier-Stokes, Fourier, and Fick equations: Dieter Straub (1996) published a book on an Alternative Mathematical Theory of Non-equilibrium Phenomena. Howard Brenner (since 2005) wrote a number of papers, like Navier-Stokes, revisited, and Bi-velocity hydrodynamics, explicitly pointing to the forces acting on the vanes of the lightmill, to thermophoresis and related phenomena. Franz Durst (since 2006) also developed modifications of the classical Navier-Stokes equations. So, Reynolds, Maxwell, and the radiometer may finally have initiated a revision of the fundamental equations of thermofluiddynamics and heat- and mass transfer.

Current and Future Needs in Electric Drive Vehicle Batteries

Batteries are considered to be a critical component for making future transportation more energy-efficient and less dependent on petroleum through hybrid, plug-in hybrid, electric, and fuel cell vehicles. In this paper, the current status of batteries for electric drive vehicle applications will be discussed. Special attention will be given to the thermal issues associated with batteries in vehicle environment. We will also discuss future needs of batteries for further development.

Thermal Management Issues in Fuel Cell Technology

Fuel cells are electrochemical energy conversion devices that convert chemical energy in fuels directly into electrical energy, without the process of combustion. As a result, they are not constrained by the thermodynamic limitations of heat engines and therefore have the potential to achieve higher efficiencies. Various fuel cell types exist, operating from room temperature to over 1000 °C. This paper focuses on two of the leading fuel cell types, namely the lower temperature (80–120 °C) polymer electrolyte membrane fuel cell (PEMFC) and the higher temperature (500–1000 °C) solid oxide fuel cell (SOFC), with particular attention paid to the importance of thermal management and heat transfer in these systems, as it is thermal transients, and the appropriate design of the thermal management sub-system, that frequently limit fuel cell system performance and durability. Two examples of research from the authors’ laboratories are given; the first relates to the measurement and modelling of heat transfer in PEMFCs; the second to the thermal management of SOFCs.

Optimization of a Pyroelectric Energy Converter for Harvesting Waste Heat

Pyroelectric energy conversion offers a novel approach for directly converting waste heat into electricity. This paper reports numerical simulations of a prototypical pyroelectric energy converter. The two-dimensional mass, momentum, and energy equations were solved to predict the local and time-dependent pressure, velocity, and temperature. Then, the heat input, pump power, and electrical power generated were estimated, along with the thermodynamic energy efficiency of the device. It was established that reducing the length of the device and the viscosity of the working fluid improved the energy efficiency and power density by increasing the optimum operating frequency of the device. Results show that a maximum efficiency of 5.2% at 0.5 Hz corresponding to 55.4% of the Carnot efficiency between 145 and 185°C can be achieved when using commercial 1.5 cst silicone oil. The maximum power density was found to be 38.4 W/l of pyroelectric material.

Effect of Rain on Evolution of Distribution of Soluble Gaseous Pollutants in the Atmosphere

We suggest a model of rain scavenging of soluble gaseous pollutants in the atmosphere. It is shown that below-cloud gas scavenging is determined by non-stationary convective diffusion equation with the effective Peclet number. The obtained equation was analyzed numerically in the case of log-normal droplet size distribution. Calculations of scavenging coefficient and the rates of precipitation scavenging are performed for wet removal of ammonia (NH3) and sulfur dioxide (SO2) from the atmosphere. It is shown that scavenging coefficient is non-stationary and height-dependent. It is found also that the scavenging coefficient strongly depends on initial concentration distribution of soluble gaseous pollutants in the atmosphere. It is shown that in the case of linear distribution of the initial concentration of gaseous pollutants whereby the initial concentration of gaseous pollutants decreases with altitude, the scavenging coefficient increases with height in the beginning of rainfall. At the later stage of the rain scavenging coefficient decreases with height in the upper below-cloud layers of the atmosphere.