Transient buoyancy-driven ventilation: Part 2. Modelling heat transfer

A new mathematical model for buoyancy-driven ventilation [Sandbach SD, Lane-Serff GF. Transient buoyancy-driven ventilation: Part 1. Modelling advection. Building and Environment, 2011] is modified to include heat transfer at the boundaries. Heat transfers at the ceiling and floor are included, using Newton’s law of cooling to model convective heat transfer between the air and the solid boundaries, Fourier’s law to model conductive heat transfer through the floor and ceiling, and a linear version of the Stefan–Boltzmann law to model radiative heat transfer from the ceiling to the floor. The effectiveness of the model was assessed using experimental results obtained in a full-scale test room. In these experiments, the vertical temperature stratification was measured using an array of T-type thermocouples. Speed measurements were obtained to estimate the ventilation flow rate (for displacement ventilation) and the velocity profile across the doorway (for doorway ventilation). Buoyancy was introduced using a twin-hob (∼2.35 kW) heat source, and in most cases a diffuse two-layer temperature stratification developed. The results from these experiments are compared with the model and existing adiabatic models. Our results indicate that the effect of heat transfer at the boundaries on the final stratification is significant and should not be ignored. Furthermore, direct comparisons between the measured and modelled results are in general very good.     

Comparison of annual energy performances with different ventilation methods for temperature and humidity control

Stratum ventilation has been proposed to accommodate elevated indoor temperatures recommended by governments in East Asia. TRNSYS is used for computation of the space cooling loads, sensible and latent, as well as system energy consumption. Typical configurations of an office, a classroom and a retail shop in Hong Kong are investigated. Desiccant dehumidification with and without solar assistance is utilized for the air treatment under displacement ventilation and stratum ventilation, while simple reheating is adopted under mixed ventilation. Compared with mixing ventilation and displacement ventilation, stratum ventilation derives its energy saving potential largely from the following five factors: the reduction in ventilation, dehumidification and transmission loads, prolonged free cooling period and increased the COP of the chillers. For the office, the year-round energy saving is found to be substantial at 20% and 40% without the need for solar energy provision when compared with displacement ventilation and mixing ventilation respectively. For the classroom and retail shop, the year-round energy saving is at about 25% and at least 37% with the aid of solar energy provision when compared with displacement ventilation and mixing ventilation respectively.     

On the efficiency of night ventilation techniques applied to residential buildings

Climatic change and heat island effect in combination with the non-proper design of buildings have increased substantially the cooling load of buildings. Night ventilation appears to be one of the more promising passive cooling techniques. Many important theoretical and experimental studies have been performed however the existing information is presented in a segmented way. The present paper analyses energy data from two hundred fourteen air conditioned residential buildings using night ventilation techniques. The specific absolute energy contribution of night ventilation has been calculated. The relation of the cooling demand of the buildings with the specific contribution of night ventilation has been investigated. It is found that the higher the cooling demand of the building, the higher the potential contribution of night ventilation under specific boundary conditions. The role of air flow rate is investigated as well. It is found that the global utilisability of the energy stored during the night increases as a function of the air flow rate and the tilt of the regression line between the energy contribution and the air flow rate increases significantly with the air flow rate applied, although the energy contribution per unit of air flow is decreasing. The whole analysis contributes towards a better understanding and evaluation of the expected energy contribution of night cooling techniques.     

General expressions for the calculation of air flow and heat transfer rates in tall ventilation cavities

Solar heated tall ventilation cavities including solar chimneys are used to enhance natural ventilation of buildings. A validated CFD model was used to predict the buoyancy-driven air flow and heat transfer rates in vertical ventilation cavities with various combinations of heat distribution on two vertical walls ranging from symmetrical to fully asymmetrical heating. The natural ventilation rate and heat transfer rate have been found to vary with the total heat input, heat distribution on the cavity walls, cavity width and height and inlet opening position. General expressions for these variables have been obtained and presented in non-dimensional terms, Nusselt number, Reynolds number, Rayleigh number and aspect ratio (H/b), as Nu = f(Ra, H/b) and Nu = f(Ra, Re) or Re = f(Ra, Nu), for natural ventilation design.     

Experimental evaluation of heat transfer coefficients between radiant ceiling and room

The heat transfer coefficients between radiant surfaces and room are influenced by several parameters: surfaces temperature distributions, internal gains, air movements.The aim of this paper is to evaluate the heat transfer coefficients between radiant ceiling and room in typical conditions of occupancy of an office or residential building. Internal gains were therefore simulated using heated cylinders and heat losses using cooled surfaces. Evaluations were developed by means of experimental tests in an environmental chamber.Heat transfer coefficient may be expressed separately for radiation and convection or as one total parameter, but this choice may lead to different considerations about thermal performance of the system. In order to perform correct evaluations, it is therefore extremely important to use the proper reference temperature.The obtained values confirm tendencies found in the literature, indicating limitations and possibilities of radiant ceiling systems improvement.     

Comparison of annual energy performances with different ventilation methods for cooling

Stratum ventilation has been proposed to cope for elevated indoor temperatures recommended by governments in East Asia. TRNSYS is used for computation of the space cooling load and system energy consumption. Typical configurations of an office, a classroom and a retail shop in Hong Kong are investigated. Compared with mixing ventilation and displacement ventilation, stratum ventilation derives its energy saving potential largely from the following three factors: the reduction in ventilation and transmission loads and increased COP of chillers. The year-round energy saving is found to be substantial at 25% and 44% at least when compared with displacement ventilation and mixing ventilation, respectively.     

Computer modeling of displacement ventilation systems based on plume rise in stratified environment

A model for displacement ventilation system based on plume rise of single point heat source was developed. The errors for temperature gradient ratio were less than 6% in most cases. Errors for temperature gradient and displacement zone height were relatively higher (up to 28.1%) which might be due to the derivation of the parameters from experimental data. Still, the errors were lower than those from design model/method of some other workers (68.5% for the temperature gradient ratio and 15.7% for the temperature difference between the supply air and at 0.1 m above floor level). With a room height of 2.4 m (common for office in Hong Kong) and design room temperature 25.5 °C defined at 1.1 m above floor level under the normal load to air flow ratio of 12,000 W/m³/s (typical values for sub-tropical region) and minimum supply temperature of 18 °C, there existed a zone capacity range from 1000 to 5000 W that stand alone operation displacement ventilation system was feasible and that the displacement zone height (minimum 2.2 m) was above normal breathing level. The feasible zone capacity range diminished with decrease in design room temperature and/or room height. In this case, the load to air flow ratio had to be reduced, resulting in a higher flow rate when compared to a mixing ventilation system, or an auxiliary cooling facility such as a chilled ceiling had to be used.     

Co‐occupant's exposure to exhaled pollutants with two types of personalized ventilation strategies under mixing and displacement ventilation systems

Personalized ventilation (PV) system in conjunction with total ventilation system can provide cleaner inhaled air for the user. Concerns still exist about whether the normally protecting PV device, on the other hand, facilitates the dispersion of infectious agents generated by its user. In this article, two types of PV systems with upward supplied fresh air, namely a chair‐based PV and one kind of desk‐mounted PV systems, when combined with mixing ventilation (MV) and displacement ventilation (DV) systems, are investigated using simulation method with regard to their impacts on co‐occupant's exposure to the exhaled droplet nuclei generated by the infected PV user. Simulation results of tracer gas and particles with aerodynamic diameter of 1, 5, and 10 μm from exhaled air show that, when only the infected person uses a PV, the different PV air supplying directions present very different impacts on the co‐occupant's intake under DV, while no apparent differences can be observed under MV. The findings demonstrate that better inhaled air quality can always be achieved under DV when the adopted PV system can deliver conditioned fresh air in the same direction with the mainly upward airflow patterns of DV.     

Theoretical and experimental investigation of impinging jet ventilation and comparison with wall displacement ventilation

This paper focuses on evaluating the performance of a new impinging jet ventilation system and compares its performance with a wall displacement ventilation system. Experimental data for an impinging jet in a room are presented and non-dimensional expressions for the decay of maximum velocity over the floor are derived. In addition, the ventilation efficiency, local mean age of air and other characteristic parameters were experimentally and numerically obtained for a mock-up classroom ventilated with the two systems. The internal heat loads from 25 person-simulators and lighting were used in the measurements and simulations to provide a severe test for the two types of ventilation systems. In addition to a large number of experimental data CFD simulations were used to study certain parameters in more detail. The results presented here are part of a larger research programme to develop alternative and efficient systems for room ventilation.     

Experimental and theoretical investigation of non-isothermal transfer in hygroscopic building materials

In this paper a modified two-dimensional Luikov model for evaluating the non-isothermal moisture migration in porous building materials was proposed. The coupled heat and moisture transfer problem was modeled. Vapor content and temperature were chosen as the principal driving potentials. The coupled equations were solved by a numerical method, which consists of a finite difference technique with a fully implicit scheme in time. Two validation experiments were developed in this study. The evolution of transient moisture distributions in both one-dimensional and two-dimensional cases was measured. A comparison between experimental results and those obtained by the numerical model proves that they are fully consistent with each other. The modified model can be integrated into a whole building heat, air and moisture transfer model.     

三维高效强化传热管传热系数的试验研究(1):蒸发强化传热

试验测试了一种可用于满液式壳管换热器的管外侧强化沸腾和强化冷凝的双效强化传热管.用热阻分离法计算了使用R134a制冷剂时制冷剂侧和水侧的表面传热系数.试验结果表明,在较大的热流密度(大于20 kW/m2)下该管较传统的强化沸腾传热管具有较大的池沸腾表面传热系数;在较大的热流密度(大于15 kW/m2)下较传统的强化冷凝管具有较大的管外冷凝表面传热系数.     

Vertical profiles of temperature and contaminant concentration in rooms ventilated by displacement with heat loss through room envelopes

The purpose of this study is to examine the effect of heat loss through walls upon the gradients of temperature and contaminant concentration in room with displacement ventilation. It is known that conduction heat loss is governed by outside temperature, heat load inside the room, supply air temperature and overall heat transfer coefficient of walls. Experiments were conducted to measure the temperature gradient and the ventilation efficiency in the room ventilated by displacement ventilation with various combinations of heat load and temperature difference between supply air and outside air. In order to simulate the change of seasons, the supply air temperature was changed instead of the outside air temperature. The effect of supply air temperature and heat generation inside the room on the temperature gradient and the concentration of tracer gas were investigated through the experiments. As a result, it turned out that the higher the heat generation rate and the lower the supply temperature, the stronger the temperature stratification and the lower the concentration in the lower zone. Additionally, ventilation heat loss turned out to be a good index for assessing the concentration in the lower zone. Temperature differences of around 3 degrees C between supply air temperature and exhaust temperature are at least needed for displacement ventilation under the conditions of the experiment presented in this paper.     

Experimental and computational investigation into suppressing natural convection in chilled ceiling/displacement ventilation environments

The combination of chilled ceiling and displacement ventilation systems can cause destruction of the displacement flow pattern in some circumstances. This paper reports on the performance of a new technique for achieving stable conditions for displacement airflow in the presence of a chilled ceiling system. The technique is based on the attachment of a honeycomb slat system to the underside of a chilled ceiling, thereby suppressing downward cool natural convection. Investigations were carried out using both computational and experimental methods for a range of typical office environment conditions. The results showed that a slat depth to width ratio of 10 could suppress the natural convection by more than 80% when the Rayleigh number reached 7 × 10⁶. This confirms that the technique is capable of minimising downward cool air currents, resulting in preservation of the displacement flow pattern in the presence of the chilled ceiling. The proposed slat system can raise the general air temperature in the space allowing some displacement flow pattern to occur. The outcome of this study is the emergence of a honeycomb slat-based approach for improving the performance, together with provision of general advice for designers as regards the combination of radiant cooling/displacement ventilation systems.     

Experimental and numerical study of room airflow under stratum ventilation

This paper investigates the air movement, air temperature profile and gaseous contaminant transportation in an individual office with stratum ventilation. The room temperature is elevated compared with conventional standards. The experimental investigation is carried out in an environmental chamber with the presence of heat generating rectangles used to simulate an occupant and a computer. Measurements of temperature, velocity, and CO₂ concentration are carried out for nine plumb lines in the chamber. Up to sixteen points are measured along each plumb line. The experimental data of the aforesaid three parameters of the individual office in warm condition under stratum ventilation are presented. The experimental data collected are used to validate a re-normalization group (RNG) k–? turbulence model used for the warm condition. The agreements between the predicted values and experimental results are acceptable, which demonstrates the feasibility of simulating indoor airflows at elevated room temperature under stratum ventilation by the RNG k–? turbulence model.     

Experimental investigation on heat transfer characteristics of heat exchanger with bubble fin assistance

ABSTRACT

Heat transfer area per unit volume (m²/m³) of heat exchangers decides the size of the heat exchangers, over the period of years heat exchangers have undergone numerous development in this aspect. One such attractive design is plate heat exchanger which is very compact and has high thermal effectiveness, whereas the flow nature of fluid through this type of heat exchangers is complex. The most common type of plate heat exchanger is chevron type which suffers from large pressure drop (Δp) at higher heat transfer rates, to overcome this problem bubble finned heat exchangers are designed. In this paper the performance of bubble finned heat exchanger is studied with single phase fluid flow condition. The comparative study of finned and no fin configuration shows that the former has 1.8 times higher rate of heat transfer at lower value of Reynold’s number, further the flow remains laminar hence the Δp is minimum.     

Experimental and numerical investigation on bare tube cross flow heat exchanger-using COMSOL

ABSTRACT

In this study, COMSOL multi-physics modelling software was used to make a computational model of a bare helical tube cross flow heat exchanger in order to simulate the temperature changes in the heat exchanger. The computational results of heat transfer are validated by using the analytical models. A conjugate convection/conduction heat transfer model was developed, which exhibited good agreement to the experiments. A different velocity of air taken into the consideration to find out the temperature distribution through the pipe and air temperature inside the duct. The temperature profile, and the overall heat transfer rate from the wall of the tube were calculated and plotted for theoretical, experimental and Numerical method using the k- conjugate heat transfer model. The model is validated through comparison with theoretical relations for single-pass cross-flow arrangements and with experimental results also. Simulation results showed good agreement with experimental values with respect to different mass flow rates.     

A numerical investigation into the effect of Windvent louvre external angle on passive stack ventilation performance

The Windvent is a commercially available passive ventilation device. The device is constructed from sheet metal and works on the principle of pressure differential. Whereby warm air rises, creating a low pressure in the receiving room, which then draws in the fresh air. This paper investigates the effect of altering the external angle of the Windvent louvres against the internal pressure and velocity within the device and the microclimate velocity. Numerical analysis is carried out using a commercial Computational Fluid Dynamics (CFD) code, to investigate the effect of various louvre angles (range 10–45°) on pressure and velocity to optimise the device performance. The results show that the louvre performance mimics that of thin airfoil from aerodynamic theory. The relationship between trailing-edge stall and delivery velocity is established. The optimum louvre angle with a prevailing wind velocity of 4.5 m/s is shown to be 35° with a stall angle of 40° illustrated. The external, performance enhancing louvre angle, determined through this investigation is subject to UK patent number 0809311.4.     

Application potential of solar air-conditioning systems for displacement ventilation

Solar air-conditioning can have higher application potential for buildings through the strategy of high temperature cooling. In recent years, displacement ventilation (DV), which makes use of the indoor rising plumes from the internal heat gains, provides a more effective supply air option than the traditional mixing ventilation (MV) in terms of both thermal comfort and indoor air quality. As it is possible to raise the supply air temperature to 19 °C for DV, it would enhance the competitive edge of the solar air-conditioning against the conventional vapour compression refrigeration. Through dynamic simulation, a solar-desiccant-cooling displacement ventilation system (SDC_DV) was developed for full-fresh-air provision, while a solar-hybrid-desiccant-cooling displacement ventilation system (SHDC_DV) for return air arrangement. The latter was further hybridized with absorption chiller (AB) to become SHDC_AB_DV, or adsorption chiller (AD) to be SHDC_AD_DV, in order to be wholly energized by the solar thermal gain. Benchmarked with the conventional system using MV, the SDC_DV had 43.3% saving in year-round primary energy consumption for a typical office in the subtropical climate; the SHDC_AB_DV had 49.5% saving, and the SHDC_AD_DV had 18.3% saving. Compared with their MV counterparts, the SDC_DV, the SHDC_AB_DV and the SHDC_AD_DV could have 42.4%, 21.9% and 30.3% saving respectively.     

Ground heat transfer: A numerical simulation of a full-scale experiment

A numerical simulation of ground-heat transfer adjacent to an experimental earth–contact structure is presented. In particular, a two-dimensional time-dependent simulation is compared directly to data measured from an experimental site over a one year period. Determination of representative thermal properties for the materials involved is explored in some detail. Indirect methods of estimating thermal conductivity and volumetric heat capacity have been described and employed. The results show good correlation between the simulated and measured thermal response. The work is viewed as a useful contribution to the overall drive to validate earth–contact simulation. However, difficulties in determining realistic initial conditions when attempting to model field conditions still remain a challenge. Further exploration of material property variations throughout the full range of climatic conditions is also needed.     

Experimental investigation of thermal and ventilation performances of stratum ventilation

Stratum ventilation has been proposed to cope for elevated indoor temperatures. Air speed, temperature and CO₂ concentration of a stratum ventilated office are investigated experimentally. The data obtained under well defined conditions and therefore can be used for validating numerical models. Thermal comfort conditions and ventilation efficiency are studied based on the experimental results of four experimental cases. Thermal comfort indices, i.e. PMV, PPD and PD are calculated from measured data. The values of these indices are found to satisfy the requirements of ISO 7730, CR 1752-1998 and ASHRAE 55-2010. In terms of thermal comfort, the two cases with supply air temperature of 21 °C are found to perform better compared with the two cases with supply air temperature of 19 °C. For all the cases, the ventilation effectiveness is close to 1.5. This ventilation method could therefore be expected to provide indoor air quality in an efficient way.