Forced convection heat transfer from a flat-plate model collector on roof of a model house

Presented in this abstract are the considerations and results of an investigation of forced convective heat loss from the surface of flat-plate model collectors flush mounted on the roof of a model residential house. Forced convection with air as the external fluid was treated.The study was motivated by the need to better understand heat transfer from the top surface of flat-plate solar energy collectors. The experimental work dealt with heat transfer from the top of rectangular parallelpipeds flush mounted on roof of a model residential house. The smallest collector was 7.62×10^–2 m long by 7.62×10^–2 m wide by 3.3×10^–2 m deep, and the largest collector was 15.875×10^–2 m long by 15.875×10^–2 m wide by 3.3×10^–2 m deep. These small scale units are expected to simulate heat loss pattern from a flat-plate solar collector placed over the roof of a residential dwelling. Experiments were performed to determine the average heat transfer coefficients for forced convective air flow over the surface of the model collector which was inclined and yawed relative to the incoming air flow. The wind speeds varied approximately from 2.5 m/sec to 15 m/sec, the flow was laminar for all wind speeds.

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Konvektiver Wärmeübergang von Flachplatten-Kollektoren auf den Dächern von Wohnhäusern im Modellversuch

Zusammenfassung Es werden Vorüberlegungen und Ergebnisse bezüglich einer Untersuchung mitgeteilt, deren Ziel es war, die Wärmeverluste infolge Zwangskonvektion von Flachplatten-Kollektoren auf den Dächern von Wohnhäusern im Kleinmodell zu bestimmen. Angeregt wurde die Studie durch den Bedarf an genaueren Berechnungsunterlagen für den Wärmeübergang an Flachplatten-Solarkollektoren. Die für das Experiment verwendeten zwei Kollektoren waren quadratisch, mit den Seitenlägen 7,62 bzw. 15,9 cm, bei jeweils 3,3 cm Tiefe. Die Versuche lieferten unter Zwangskonvektion mittlere Wärmeübergangskoeffizienten an den Oberflächen der Modellkollektoren, welche zur ankommenden Luftströmung sowohl geneigt als auch gedreht orientiert waren. Bei laminarer Strömung lagen die Anströmgeschwindigkeiten zwischen 2,5 and 15 m/s.

     

Investigation of a flat-plate oscillating heat pipe with Tesla-type check valves

Tesla-type check valves were integrated into a flat-plate oscillating heat pipe (FP-OHP) in order to promote and sustain a desired circulatory flow to increase overall thermal performance. Using neutron radiography, gray-scale images capturing the internal flow behavior within two bottom-heated copper FP-OHPs - one with Tesla-type valves (TV FP-OHP) and one without - both charged with water at a filling ratio of 70% - were collected. With the Tesla-type valves installed in the adiabatic section of the TV FP-OHP, it was found that circulation in the desired direction was promoted and that this promotion increased with heat input. The TV FP-OHP consistently possessed a lower thermal resistance than its counterpart without check valves. The percent-reduction in thermal resistance was on-the-order of 15-25% depending on the power input. Implementation of Tesla-type check valves is a promising means for circulatory flow rectification within an OHP, but future research is needed to further optimize valve design, quantity, and alignment.     

Investigation of temperature distribution in a piecewise-homogeneous seam with pressed-in hot fluid

Temperature distribution in a piecewise-homogeneous finiteseam exposed to hot fluid and the effect of inhomogeneity in the permeability and thermal properties of separate zones of a seam on the redistribution of temperature fields for flat parallel or axial flow of a pressed-in hot liquid are studied. The differential equations which describe the process of temperature distribution in accordance with [1] are solved for various initial and boundary conditions. Exact analytic formulas are obtained which are useful in numerical computations. The problem under consideration is related to important engineering problems in hydrology, geothermy, as well as in the development of oil or gas fields [2–4, 5].     

Experimental and computational study of the developed flow field in a flat plate integrated collector storage (ICS) solar device with recirculation

The flow structure in a flat plate integrated collector storage device, with recirculation of the storage water, is studied experimentally and theoretically. To facilitate flow visualization, an experimental device was constructed by transparent material (Plexiglas). Flow velocities and fluctuations are measured, using a LDV system. A three-dimensional CFD-model was developed using the FLUENT code. The standard k–ω model is selected as the most appropriate. The model is validated, with good agreement, against experimental measurements. Furthermore, copper tubes, in the form of embedded heat exchanger, are placed inside the device and another similar 3D model was developed. The model was used to examine the behavior of the system, when the service water enters the heat exchanger, thus being indirectly heated by the stored hot water. It is shown that the outlet temperature of the service water is enough higher, when recirculation occurs.     

Numerical and experimental studies of temperature field characteristic inside an irregularly-shaped cavity of a space environment simulator system

Numerical and experimental methods were used to explore temperature and pressure distributions inside an irregularly-shaped cavity of a novel three-dimensional space environment simulator (SES) system. In order to obtain better temperature and pressure distributions, a plenum chamber and airflow diffusion perforated plate were adopted. Three-dimensional heat and mass transfer characteristics were analyzed using the Standard k–ε turbulence model. Simulation results revealed that the temperature and pressure distributions were greatly improved with improved diffusion configuration design, the temperature gradient decreased from 5 K to 1 K, and the pressure gradient decreased to 0.5% of the former value. Based on the simulation results, an improved experimental system for simulating space environment was set up. This experiment system could supply airflow with temperature ranging from 193 K to 353 K for simulating the real space environment. Experimental results showed that the temperature and pressure fields had smaller gradients across the surface and the inner cavity, which agreed considerably with the numerical results. The results of this study present useful information for the design of similar cavity structure.     

Natural convection in partitioned enclosed spaces of solar collector

The two-dimensional, steady, laminar natural convection phenomena in partitioned enclosure of solar collector has been studied numerically. Heat conduction along the partition is considered. An iterative finite-difference scheme is employed to solve the governing equations in the flow field. The effects of Rayleigh number, thermal conductivity ratio, partition angle, tilt angle, and aspect ratio on both the local and average heat transfer coefficients of the solar collector have been discussed. The range of Rayleigh number tested was up to 5 × 10⁴, the thermal conductivity ratio of 4.5 and 30, partition angle from 10 deg to 170 deg, tilt angle from 10 deg to 90 deg, and aspect ratio varied between 0.2 and 10. The results indicate that the convective heat transfer is strongly affected with the aspect ratio of the enclosures.

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Freie Konvektion in unterteilten Kammern von Solarkollektoren

Zusammenfassung Die zweidimensionale, stetige, laminare freie Konvektion in unterteilten Kammern von Solarkollektoren wurde numerisch untersucht. Die Wärmeübertragung entlang dieser Kammern wurde betrachtet. Ein iteratives Finite-Differenzen-Schema wurde angewandt um die Gleichungen, welche das Strömungsfeld beschreiben, zu lösen. Der Einfluß der Rayleigh-Zahl, der thermische Leitfähigkeit, des Kammerwinkels, des Neigungswinkels und der Längenverhältnisse auf die örtlichen und durchschnittlichen Wärmeübertragungskoeffizienten des Solarkollektors wurde diskutiert. Der Bereich der Rayleigh-Zahl erstreckte sich bis zu 5 × 10⁴, das Verhältnis der thermischen Leitfähigkeit betrug 4.5 und 30, der Kammerwinkel lag zwischen 10° und 170°, der Neigungswinkel zwischen 10° und 90° und das Längenverhältnis variierte zwischen 0.2 und 10. Die Ergebnisse beinhalten, daß die konvektive Wärmeübertragung sehr stark durch das Längenverhältnis der Kammern beeinflußt wird.

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Nomenclature a slope of the partition with respect to the horizontal - A H/L=cell aspect ratio - A _w t/L=wall aspect ratio - g acceleration due to gravity - h local heat transfer coefficient - average heat transfer coefficient - H cell length - k thermal conductivity of fluid within the cell - k _w thermal conductivity of the cell wall - L plate spacing - Nu _f h L/k=local cell Nusselt number - L/k=average cell Nusselt number - overall average Nusselt number - qL/k _w t(T _h–T _c)=average wall Nusselt number - Pr /=Prandtl number - q heat transfer in the cell wall from the hot to cold plate per unit depth - Ra g L ³ T/=Rayleigh number - R _k k _w/k=ratio of wall thermal conductivity to that of the fluid - t thickness of cell wall - T _c cold plate temperature - T _f temperature in cell - T _h hot plate temperature - T _w temperature in cell wall - u, U dimension and dimensionless velocities inx-direction - v, V dimension and dimensionless velocities iny-direction - x distance measured from the bottom of the cell (Fig. 1) - X x/L=normalized distance ofx - y distance measured from hot plate (Fig. 1) - Y y/L=normalized distance ofy - x ₁ distance measured in wall (Fig. 1) - X ₁ x/L=normalized distance ofx ₁ Greek symbols thermal diffusivity of fluid - coefficient of volumetric expansion of fluid - partition angle with respect to the hot plate - _f T _f–T _c/T _h–T _c=dimensionless temperature in cell - _w T _w–T _c/T _h–T _c=dimensionless temperature in cell wall - kinematic viscosity of fluid - enclosure tilt angle from horizontal - dimensional vorticity - L ²/=dimensionless vorticity - dimensionless streamline     

‘Settling time’ and ‘memory’: two concepts to simplify the dynamic analysis of the flat-plate collector

Concepts of ‘settling time’ and ‘memory’ in a solar collector are introduced and explained; they are shown to characterize the collector and the fluid-flow collectively. These two characteristics are then used to develop a simplified method for the calculation of the fluid temperature within a flat-plate collector. Under all conditions, the proposed method can be used with any arbitrary degree of accuracy. The temperature is given, in a normalized form, as a function of both time and position along the collector. The numerous design and operating variables are lumped in a limited number of dimensionless groups     

Thermocapillary migration of a fluid droplet inside a drop in a space laboratory

The thermocapillary migration of a fluid droplet located inside a liquid drop in a space laboratory is analyzed. The quasi-static momentum and energy equations are solved at the instant when the droplet passes the center of the drop. Results are presented for prescribed axisymmetric distributions of temperature on the drop surface.     

Formation of the free surface of a viscous fluid volume inside a rotating horizontal cylinder

Two-dimensional viscous flow with a free surface in a horizontal cylinder rotating at a constant speed is investigated numerically using the boundary element method. It is shown that in the initial stage of rotation of the cylinder four different variants of the behavior of the free surface can be realized in the stage of transition from horizontal to steady-state form.     

Visualization of natural convection inside shallow solar stills

 Visualization techniques are employed to contribute to the understanding of the mechanisms responsible for heat and mass flow between the water basin and the condensing cover inside a shallow solar still. Laser sheets and tracers were employed to record images, where the dominant nature of convection in cell patterns was observed. Received: 1 February 1997/Accepted: 2 February 1998     

Model of coupled fluid flow in a reservoir and inside a horizontal well

A model and a numerical-analytic method of solving problems of single-phase adjoint fluid flow in a porous medium and in a horizontal well are proposed.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 5, pp. 112–117, September–October, 1996.     

Influence of spatial modulation of the temperature field on the stability of two-dimensional steady flow in a horizontal layer of fluid

A study is made of the stability of the steady periodic regime that arises in a horizontal layer of fluid in the presence of spatial modulation of of the temperature on the solid bottom boundary. The upper free boundary of the layer is in contact with the atmosphere. The fundamental resonance values of the wave number of the modulation are found; there are five of them. If the temperature of the lower boundary of the layer is constant, and the temperature gradient is not too large, the fluid is in equilibrium. When the temperature gradient passes through the critical value, the equilibrium ceases to be stable, and steady convection develops in the fluid [1]. In the presence of spatial modulation of the temperature on the lower boundary of the layer the fluid cannot be in equilibrium, and a spatially periodic steady regime is established in it. The aim of the present paper is to find the critical values of the temperature gradient at which this fundamental steady regime becomes unstable and a secondary steady regime develops in the fluid. An analogous problem for the case when both boundaries of the layer are free surfaces and without allowance for the influence of the atmosphere has been solved by Vozovoi and Nepomnyashchii [2].     

Performance study of unglazed cylindrical solar collector for adsorption refrigeration system

In the present communication, the unglazed cylindrical solar adsorber module is suggested for refrigeration and theoretical models for the heat and mass transfer in the cylindrical adsorber with heat balance equations in the collector components have been developed. It has been found that, both the SCP and COP_solar raises with increasing the evaporation temperature and drop off with the increase of the condensation temperature. The COP_solar increased from 0.15 to 0.52 with the increase of the total solar energy absorbed by the collector while the COP_cycle varied in the range of 0.57–0.73. The efficiency of unglazed solar collector varied from 36 to 44 %. The cost of current unglazed adsorption refrigeration system is compared with the glazed system, and it is 33 to 50 % less than the cost of glazed system.     

Temperature distribution inside hot-water storage tanks of solar collectors

An analytical-experimental investigation into the temperature field inside the hot-water storage tank of a solar collector is carried out. A transient two-dimensional semi-infinite cylindrical length model with time and space boundary conditions dependency is selected. Conduction and convection heat transfer modes in the axial direction together with conduction in the radial direction are considered. Good agreement between theoretical and experimental predictions is verified. The radial direction temperature dependency disappears for axial lengths greater than one quarter of the tank depth for most practical operating conditions especially for low inflow velocities and low inlet to outlet temperature ratios. The axial conduction term in the governing equation can be dropped out for inflow velocities greater than a certain critical value without disturbing the theoretical consequences.Temperature profiles in the axial direction can be assumed to be linear especially for high inflow velocities as well as low temperature differences at inlet and outlet of the storage tank.

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Temperaturverteilung in einem Heißwasser-Speichertank eines Sonnenkollektors

Zusammenfassung Eine analytisch-experimentelle Untersuchung des Temperaturfeldes in einem Heißwasser-Speichertank eines Sonnenkollektors wurde durchgeführt. Es wird ein zweidimensionales instationäres halbunendliches zylindrisches Modell mit zeitlichen und örtlichen Randbedingungen ausgewählt. Wärmeübertragung durch Leitung und Konvektion in axialer Richtung zusammen mit Leitung in radialer Richtung werden betrachtet. Gute Übereinstimmung zwischen theoretischen und experimentellen Ergebnissen werden festgestellt. Die Temperaturabhängigkeit in radialer Richtung verschwindet für axiale Längen größer als ein Viertel der Tanktiefe für die meisten praktischen Anwendungen, speziell für niedrige Einströmgeschwindigkeiten und für ein niedriges Ein- und Auslaß-Temperaturgefälle. Der Term, der die axiale Leitung beschreibt, kann für Einströmgeschwindigkeiten, die eine bestimmte kritische Größe überschreiten, in der Gleichung eliminiert werden, ohne die theoretischen Folgerungen zu stören. Die Temperaturprofile in axialer Richtung können speziell für hohe Einströmgeschwindigkeiten und kleine Temperaturunterschiede an Ein- und Auslaß des Lagertanks als linear angenommen werden.

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Nomenclature a radius of the tank, m - A _c collector area, m² - C _p constant pressure heat capacity, kJ/(kg·°C) - F _R heat removal factor - F collector flow factor - h wall heat transfer coefficient, W/(m²·°C) - J _o zero-order Bessel function - J ₁ first-order Bessel function - K thermal conductivity, W/(m·°C) - L length of the tank - r radial direction - t dimensionless time - T dimensionless temperature - T _p,m mean plate temperature, °C - ¯T temperature, °C - T¯ ₁ maximum temperature, °C - T¯ ₂ minimum temperature at inlet, °C - T¯ _f,i inlet fluid temperature to the collector, °C - U _t top loss coefficient, W/(m² · °C) - ¯V _r radial velocity component, m/sec - ¯V _i initial temperature in the tank, °C - ¯V _z axial velocity component - V velocity component, m/sec - V dimensionless axial constant value of velocity - W _i heat generation, kW/m³ - Z dimensionless axial direction - thermal diffusivity, m²/sec - _n roots of first-order Bessel function - impulse function - absolute value of viscosity - Laplace transform of temperature density - density, kg/m³     

Stokes flow inside and around a fluid drop distorted by the presence of a tube wall

Using a quadratic optimisation process to satisfy the boundary conditions, the drag coefficient and the flow patterns inside and outside a fluid drop translating axially in a tube have been determined accurately even when the drop is clearly elongated. Convincing comparisons with experimental results (flow visualizations, velocity and drag measurements) are presented. The effects of the viscosity and density ratios are examined.     

Optical studies of local flow behaviour of a non-Newtonian fluid inside a porous medium

In this study the local flow of a polymeric solution inside a porous medium is studied visually. While the flow is quite uniform for low volume flow rates it shows pronounced nonuniformity for higher volume flow rates. That is to say, only certain preferred passages are taken and these passages change in time (they fluctuate). This flow irregularity is the reason for increased resistance. Received: 28 April 1997 Accepted: 30 December 1997     

Estimate of the temperature field in a compressible medium

Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, No. 1, pp. 23–26, January–February, 1992.     

Preliminary results of the spilling effects on the oscillation of a pipe with compressible fluid inside

It is known to all,the spilling of pipeline may cause serious problems,especially when the pipe conveying petroleum,natural gas or other toxic substance.There are countless accidents during past century.Once the spilling occurs,the vibration of the pipe would aggravate spill situation and even result in crack of the pipe.The consequence will be more severe when the fluid inside is compressible.To prevent the detriment of the spilling model is developed by assuming the leakages as orifices or nozzles and a 2-D vertical simply supported pipe is selected to analyze the phenomena of the oscillation.Combining these two models,the oscillation model for the pipe with leakage is set up and the spilling effect is analyzed by numerical method.The amplitude of the pipe oscillation and the normal stress enlarge as the internal velocity increased,while the shear stress changes very little.