Impact of direct solar irradiance on heat transfer behind an open-jointed ventilated cladding: Experimental and numerical investigations

This paper is based on the study of an experimental wooden-framed house, equipped with a ventilated cladding, located in France. The focus is on investigating the heat transfer taking place in the ventilated air gap behind the cladding and on estimating their impact on heat transfer in the insulated part of the walls.Preliminary CFD simulation made it possible to describe the airflow in the ventilated air gap when buoyancy was acting as the main driving force. It was found that the airflow velocity could reach 0.8 m s^?1, when exposed to 570 W m^?2 solar irradiance. This was verified experimentally. A correlation to assess airflow rate in the air gap behind the cladding has been developed based on temperature differences and on experimental conclusions. The correlation is adapted to energy performance simulation and efficiently represents the average cavity airflow at different height for various irradiance and temperature conditions.     

A validated model of naturally ventilated PV cladding

A simplified method has been derived for use in the estimation of the flow rate in naturally ventilated PV cladding for buildings. The method is based on a one-dimensional ‘loop analysis’ in which the buoyancy forces are balanced by the pressure drops due to friction. Wind effects at the entrance and exit are also taken into account. The procedure yields the mass flow rate and temperature rise directly by the solution of a simple cubic equation and therefore is straightforward and simple enough to be put on a spreadsheet. This methodology allows the designer to explore various potential PV configurations at little expense and hence to focus on those designs which warrant further detailed analysis, perhaps coupled to a full building simulation package. In this paper, the fundamental theory behind the loop analysis is described. The hypothesis tested is that the form and values for the friction factors and internal heat transfer coefficients for the buoyancy driven case are the same as those for forced convection in ducts. Next, the experimental rig is discussed with which the first validation exercises are carried out for the no-wind case, using an emulation of the simple single stack PV cladding arrangement. The two key parameters are identified using the measurement error weighted least squares linear regression. Overall excellent agreement between the modelled and measured mass flow rates is seen; the hypothesis is therefore valid. A general model is then derived to describe the thermal behaviour of building-integrated PV with natural ventilation cooling for use in a wide variety of design and validation exercises.     

Experimental evaluation of ventilated glazing performance in Hong Kong

Window glazing affects much the indoor environment and the energy use in buildings. While double glazing has better thermal performance than single glazing, the airflow window options carry additional advantage of directly removing the absorbed solar energy in glass panes. This paper reports an experimental study in Hong Kong in evaluating the thermal/energy performance of the above‐mentioned glazing systems. A new approach of using numerical simulation technique to improve the quality of experimental analysis was introduced. Our findings show that the natural‐ventilated glazing system has a better thermal performance than the double‐glazing system since the latter received 13.6% more convective heat gain. The main advantage of the exhaust‐ventilated glazing system lies in the decrease of convective heat transfer to 34.3% of the double glazing, and 19.4% of the single‐glazing types. The results showed that the ventilated glazing schemes in association with daylight utilization could lead to substantial electricity savings in the office environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental evaluation of barrier walls for risk reduction of unintended hydrogen releases

Hydrogen-jet flames resulting from ignition of unintended releases can be extensive in length and pose hazards associated with radiation and impingement onto objects, combustible materials and people. Depending on the leak diameter and source pressure, the resulting consequence distances can be unacceptably large. One possible mitigation strategy to reduce exposure to jet flames is to incorporate barriers around hydrogen storage and delivery equipment. While reducing the extent of unacceptable consequences, the walls may introduce other hazards if not properly configured. An experimental program has been implemented to better characterize the effectiveness of barrier walls at risk mitigation. This paper describes the experiments and presents results obtained for various barrier configurations. The measurements include flame deflection using standard and infrared video, high-speed movies (500 fps) to study initial flame propagation from the ignition source, overpressure levels due to ignition, wall deflection, radiative heat flux, and gas and wall temperatures. The various barrier designs are evaluated in terms of their mitigation effectiveness for the associated hazards present. The results show that barrier walls are effective at deflecting flames in a desired direction. While barrier walls can result in increased overpressures and radiative heat flux in the vicinity of the wall, they can also attenuate the effects of these hazards in surrounding areas if properly implemented.     

Performance evaluation of mixed convection in an inclined square channel with uniform temperature walls

Numerical analyses were performed for the effect of inclined angle on the mixing flow in a square channel with uniform temperature walls (T_w = 30 °C) and inlet temperature (T₀ = 10 °C). Three-dimensional governing equations were solved numerically for Re = 100, Pr = 0.72 and various inclined angles (from ?90° to 90°). Three-dimensional behavior of fluid in a channel was examined for each angle. Thermal performance was evaluated using the relationship between Nusselt number ratio and pressure loss ratio with and without buoyancy induced flow as a parameter of inclined angles. High heat transfer and low pressure loss region was from ?15° to ?60° in thermal performance using mean Nusselt number ratio.     

不同外墙保温方式对墙体热工性能的影响及分析

通过对墙体的温度分布及内表面温度的分析和计算,对比分析了外保温与内保温两种外墙保温方式。研究表明外墙外保温方式在提高室内舒适度、减少墙体开裂、消除热桥影响及降低能耗等方面优于内保温方式。     

General model to build awnings and external walls with optimum shading interaction

A general model is proposed to optimize the shading interaction between an awning and an external wall that project shadows on the facade of a construction. The original method was presented by the authors in Renewable Energy 28 (2003): 111, for a particular shading proposal. They emphasize the advantage of a one-dimensional approach to solve the problem of this interaction, analyzing the shadows projected by these elements from a side view. The generalized method, presented in this paper, is useful for architects to design shelter projects with more alternatives in the use of such shading devices. The general model also permits study and evaluation of any strategy chosen by the builder to shadow a facade, with an emphasis on windowpane shading, through graphical outputs of the solar performance.     

Optimum insulation thickness of external walls for energy saving

Most of energy is used up to space heating in the cold regions of Turkey. Insulation in external walls of buildings has been gaining much more interest in recent years not only for the environmental effect of the consumed energy but also the high cost of the energy. Therefore, the optimum insulation thickness was investigated in this study for the coldest cities of Turkey like Erzurum, Kars and Erzincan. The optimization is based on the life cycle cost analysis. As a result considerable energy saving is obtained when the optimum insulation thickness is applied. Savings up to 12.113 $/m² of wall area can be maintained for Erzurum.     

Experimental study of thermal stratification in ventilated confined spaces

Within the displacement ventilation (DV) field, this paper presents an experimental study of thermal stratification in a ventilated room in which there is a heat source. The ventilation system maintains plume development up to a limiting height, above which the air stream is mixed. Temperature stratification arises within the test room. Its interface is of significant thickness and its position is identified by that of the maximum standard deviation of temperature fluctuations. The change in the height of this interface is studied under different experimental conditions. Results confirm the laws already obtained for zones close to and far away from the source and they also allow a law governing the transition between these two zones to be proposed.     

Optimization of insulation thickness for external walls using different energy-sources

In countries that import most of their energy, like Turkey, energy saving and the effective usage of energy become much more important. Energy consumption for heating is too high in Turkey because buildings have almost no insulation. Also the high prices of heating energy in Turkey, emphasize the need for energy saving. Therefore, the optimum insulation-thickness of the external wall for the five different energy-sources (coal, natural gas, LPG, fuel oil and electricity) and two different insulation materials (expanded polystyrene, rock wool) are calculated for Denizli. The optimization is based on a life-cycle cost analysis. According to the results, the optimum has been obtained by using coal as the energy source and expanded polystyrene as the insulating material. When the optimum insulation-thickness is used the life cycle saving and payback period are 14.09 $/m² and 1.43 years, respectively.     

Determination of optimum insulation thickness of external walls with two different methods in cooling applications

Thermal insulation is one of the most effective energy conservation for the cooling applications. For this reason, determination of the optimum thickness of insulation and its selection is the main subject of many engineering investigations. In this study, the optimum insulation thickness on the external walls in the cooling applications is analyzed based on two different methods used to determine annual energy consumption. One of the methods is the degree-hours method (Method 1) that is the simplest and most intuitive way of estimating the annual energy consumption of a building. The other is the method (Method 2) which using the annual equivalent full load cooling hours operation of system. In this paper, a Life Cycle Cost (LCC) analysis is used to evaluate accuracy of these methods, and the results are compared. The results show that the life cycle savings are overestimated by up to 44% in Method 2, while the optimum insulation thickness and payback period are respectively overestimated by up to 74% and 69% in Method 1.     

Effect of inlet and outlet location on the mixed convective cooling inside the ventilated cavity subjected to an external nanofluid

In this paper, mixed convection flow and temperature fields in a vented square cavity subjected to an external copper–water nanofluid are studied numerically. The natural convection effect is attained by heating from the constant flux heat source on the bottom wall and cooling from the injected flow. In order to investigate the effect of inlet and outlet location, four different placement configurations of the inlet and outlet ports are considered. In each of them, both the inlet and outlet ports are alternatively located either on the top or the bottom of the sides and external flow enters in to the cavity through an inlet opening in the left vertical wall and exits from another opening in the opposite wall. The remaining boundaries are considered adiabatic. The governing equations have been solved using the finite volume approach, using SIMPLE algorithm on the collocated arrangement. The study has been carried out for the Reynolds number in the range of 50 ≤ Re ≤ 1000, with Richardson numbers 0 ≤ Ri ≤ 10 and for solid volume fraction 0 ≤ ? ≤ 0.05. Results are presented in the form of streamlines, isotherms, average Nusselt number. In addition, the effects of solid volume fraction of nanofluids on the hydrodynamic and thermal characteristics have been investigated and discussed. The algorithm and the computer code have been also compared with numerical results in order to verify and validate the model.     

Magnetoconvection in an enclosure with partially active vertical walls

Magnetoconvection of an electrically conducting fluid in a square cavity with partially thermally active vertical walls is investigated numerically. The active part of the left side wall is at a higher temperature than the active part of the right side wall. The top, bottom and the inactive parts of the side walls are thermally inactive. Nine different combinations of the relative positions of the active zones are considered. The governing equations are discretized by the control volume method with QUICK scheme and solved numerically by SIMPLE algorithm for the pressure–velocity coupling together with underrelaxation technique. The results are obtained for Grashof numbers between 10⁴ and 10⁶, Hartmann numbers between 0 and 100 and Prandtl numbers 0.054–2.05. The heat transfer characteristics are presented in the form of streamlines and isotherms. The heat transfer rate is maximum for the middle–middle thermally active locations while it is poor for the top–bottom thermally active locations. The average Nusselt number decreases with an increase of Hartmann number and increases with an increase of Grashof number. For sufficiently large magnetic field Ha = 100 the convective mode of heat transfer is converted into conductive mode in the low region of Grashof number than in the high region.     

Experimental comparison of latent and sensible heat thermal walls

Three thermal walls have been examined with the same test-cell. The complete experiment, for the three walls, lasted 3 yr. The test-cell was thoroughly instrumented to permit the energy balance on the various parts of the cell (thermal wall, inner room, front glazing) to be checked against real data. Moreover the dynamical loading of the test-cell was varied to simulate various couplings between the wall and the enclosure. This simulation was made with the help of two flat plate air-air heat exchangers covering totally the top and opposite inside walls of the cell. The simulation allowed the performance of thermal walls coupled to a back room to be assessed.

The energy yield of the walls and the temperature variations of the inside room are compared for a 40 cm thick concrete wall, an 8 cm thick hard paraffin wall, and an 8 cm thick soft paraffin wall. The advantage of the paraffin walls over the concrete is a mass, -including containers, one-twelfth smaller, thus much better suited to a retrofit.     

严寒地区农村住宅外墙节能优化研究

对大庆市杜蒙县杏树岗村的农宅外墙节能进行研究,发现农村住宅外墙大多采用无保温37黏土砖墙,采暖期的室内舒适度低,能耗高。在Ecotect中建立建筑模型,通过逐时温度曲线和逐月能耗模拟2个方面对农宅的外墙进行能耗分析,提出外墙保温层厚度设计最优值,得出大庆市农宅的外墙采用55 mm聚苯乙烯泡沫板作为外保温系统最为经济和节能,为寒地农宅室内热环境优化提供指导。     

Experimental performance evaluation of an ammonia-fuelled microchannel reformer for hydrogen generation

Microchannel reactors appear attractive as integral parts of fuel processors to generate hydrogen (H₂) for portable and distributed fuel cell applications. The work described in this paper evaluates, characterizes, and demonstrates miniaturized H₂ production in a stand-alone ammonia-fuelled microchannel reformer. The performance of the microchannel reformer is investigated as a function of reaction temperature (450–700 °C) and gas-hourly-space-velocity (6520–32,600 Nml g_cat⁻¹ h⁻¹). The reformer operated in a daily start-up and shut-down (DSS)-like mode for a total 750 h comprising of 125 cycles, all to mimic frequent intermittent operation envisaged for fuel cell systems. The reformer exhibited remarkable operation demonstrating 98.7% NH₃ conversion at 32,600 Nml g_cat⁻¹ h⁻¹ and 700 °C to generate an estimated fuel cell power output of 5.7 W_e and power density of 16 kW_e L⁻¹ (based on effective reactor volume). At the same time, reformer operation yielded low pressure drop (<10 Pa mm⁻¹) for all conditions considered. Overall, the microchannel reformer performed sufficiently exceptional to warrant serious consideration in supplying H₂ to fuel cell systems.     

Experimental evaluation of solar thermosyphons with heat exchangers

The advantages of solar thermosyphons in terms of simplicity, reliability and cost have long been recognized. Recent studies have also shown their thermal performance to be comparable with that of equivalent active systems. When pump power is considered, the energy savings of domestic hot water thermosyphons can be significantly superior to active systems. In spite of these advantages, use of solar thermosyphons in the United States is almost negligible compared to their widespread use in other countries. A major limitation to the use of thermosyphons in the United States is lack of effective, reliable freeze protection. One technique for reliable, passive freeze protection is to use a heat exchanger in the storage tank and a nonfreezing fluid in the collector. Previous analytical work indicates that the performance penalty for these systems with practical-sized heat exchangers may be small enough to make these systems economically feasible. A full-scale, residential-size test facility has been constructed for testing this concept and validating the theoretical models. This paper describes results of testing comparing the performance of a horizontal tank with and without heat exchanger to a baseline case of a vertical tank without heat exchanger. An analytical expression for a “heat exchanger penalty factor” for these systems is derived and compared with the experimental results.     

Heat pump assisted distillation. III: Experimental studies using an external heat pump

An experimental study has been carried out on a continuously operated pilot fractional distillation column equipped with an external heat pump. The distillation column was a 15 cm diameter glass unit containing eleven single bubble cap plates. A methanol-water mixture was fed to the column and the heat pump working fluid was R114. The actual coefficient of performance (COP)_A of the heat pump increased with an increase in the mass flow rate of the working fluid. A maximum (COP)_A value of 4–3 was obtained with a gross temperature lift of 41–3°C. The performance of two reciprocating compressors was compared. The experiments have shown that continuous heat pump assisted distillation using an external working fluid can greatly reduce the energy used in a distillation process. No control problems were encountered in the experiments.     

Solar energy absorption efficiency of an ellipsoidal receiver-reactor with specularly reflecting walls

An ellipsoidal cavity-receiver with specularly reflecting inner walls, in which the reactor component is positioned at one focal point and the aperture at the other, may be useful in solar applications. Most of the incident radiation from a solar concentrator should reach the reactor directly or after one reflection from the cavity walls. Because the source (aperture) and sink (reactor) have finite areas, the ellipsoidal reflector no longer conveys all of the entering radiation into the reactor; some radiation entering the cavity does not reach the target after one reflection and is eventually absorbed by the cavity walls after multiple reflections or escapes through the aperture. We have examined the conditions for which this radiation loss becomes significant and have estimated the effects on the energy-collection efficiency of the system.     

Experimental investigation of hydrogen jet fire mitigation by barrier walls

Hydrogen jet flames resulting from ignition of unintended releases can be extensive in length and pose significant radiation and impingement hazards. One possible mitigation strategy to reduce exposure to jet flames is to incorporate barriers around hydrogen storage and delivery equipment. While reducing the extent of unacceptable consequences, the walls may introduce other hazards if not properly configured. This paper describes experiments carried out to characterize the effectiveness of different barrier wall configurations at reducing the hazards created by jet fires. The hazards that are evaluated are the generation of overpressure during ignition, the thermal radiation produced by the jet flame, and the effectiveness of the wall at deflecting the flame.The tests were conducted against a vertical wall (1-wall configuration), and two “3-wall” configurations that consisted of the same vertical wall with two side walls of the same dimensions angled at 135° and 90°. The hydrogen jet impinged on the center of the central wall in all cases. In terms of reducing the radiation heat flux behind the wall, the 1-wall configuration performed best followed by the 3-wall 135° configuration and the 3-wall 90°. The reduced shielding efficiency of the three-wall configurations was probably due to the additional confinement created by the side walls that limited the escape of hot gases to the sides of the wall and forced the hot gases to travel over the top of the wall.The 3-wall barrier with 135° side walls exhibited the best overall performance. Overpressures produced on the release side of the wall were similar to those produced in the 1-wall configuration. The attenuation of overpressure and impulse behind the wall was comparable to that of the three-wall configuration with 90° side walls. The 3-wall 135° configuration’s ability to shield the back side of the wall from the heat flux emitted from the jet flame was comparable to the 1-wall and better than the 3-wall 90° configuration. The ratio of peak overpressure (from in front of the wall and from behind the wall) showed that the 3-wall 135° configuration and the 3-wall 90° configuration had a similar effectiveness. In terms of the pressure mitigation, the 3-wall configurations performed significantly better than the 1-wall configuration.