Evaluation of LES and RANS CFD modelling of multiple steady states in natural ventilation

This paper reports research carried out with the aim of evaluating and comparing the performance of Large Eddy Simulation (LES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS) modelling for predicting the multiple steady states observed in experiments on a buoyancy-driven naturally ventilated enclosure. The sub-grid scales of the flow have been modelled using a Van Driest damped Smagorinsky sub-grid scale model in the case of LES and an RNG k-ε turbulence model has been used for URANS. A novel mesh design strategy was introduced to design the LES mesh to identify an optimum 'well-resolved' mesh, assuming that the flow investigated is free-shear dominated. It was found that the URANS solution eventually settled down into a permanent steady state, displaying no evidence of continuing instabilities or periodic unsteadiness. Both URANS and LES solutions captured the existence of three steady states as observed in experimental studies. However, LES was more accurate in predicting the temperatures inside the enclosure compared to URANS. In the URANS solutions, it was observed that for smaller lower opening areas the average indoor temperature had noticeable discrepancies when compared with experimental results. Unlike URANS, LES correctly predicted different steady state temperatures for different opening areas and the time to reach steady state agreed closely with theoretical predictions

Investigating heat loss through vestibule doors for a non-residential building

The aim of this study was to investigate the effects of air flow movement through high use front entrance doors of a college building with large flows of people. The objectives were to visualize and quantify the resultant energy losses through the entrance doors, coupled with investigating any potential improvements that can be obtained through improved design. The findings of the study suggest that the heat loss from the front entrance design can contribute to up to 2.8% of the buildings’ energy loads. It was also seen that a vestibule creates a tunnel effect for cold ambient air to enter the building without hot air escaping from the vestibule. Rather hot stale air exits through openings at the ceiling height. Potential solutions with entrance design are investigated and their results compared to the outcomes of a similar model designed using Computational Fluid Dynamics (CFD)

Modelling buoyant thermal plumes in naturally ventilated buildings

The aim of the work reported in this paper was to evaluate the performance of Large Eddy Simulation (LES) for modelling natural ventilation driven by twin plumes. The flow is characterised by an interface height which separates the warm buoyant air above from the cooler air below, and a merging height for coalescence of the two plumes. Comparison between the LES predictions and theory for the interface height and volume flow rate in the merged plumes is good, giving confidence that LES has potential for modelling this important class of flows

Investigating performance of solid desiccant dehumidification in an evaporative cooling system

Desiccant evaporative cooling (DEC) systems are still at early stage of utilisation and commercialise development. The primary focus of this study is to analyse the simulated performance of DEC in hot and humid climates. Four configurations of one-stage solid desiccant dehumidifiers with two-stage evaporative cooling systems have been simulated using TRNSYS simulation software for a typical test room. The psychrometric process and the coefficient of performance (COP) for each configuration is presented and configurations for the most efficient system have been identified

Performance assessment of Fanger’s PMV in a UK residential building in heating season

Traditionally there are two approaches to thermal comfort studies in the indoor environment. The first approach is to conduct tests in fully controlled climate chambers located in laboratories which help in maintaining desired environmental conditions for the experiments. However, the thermal physics of climate chambers are very different to that of real buildings. Additionally, the numbers of participants in such studies are also limited. The alternate/second approach is to place sensors and collect data in a set of homes and offices over a period of time where researchers have virtually no control on the thermal environment. This approach does involve a large set of participants however the large variations in thermal environmental parameters make the data not very reliable to elucidate trends. This paper reports on an original approach that combines the advantages of both these methods. In this research thermal comfort studies were conducted in a test house representative of a real residential building and a large set of participants. The thermal environmental parameters and heating strategies inside the test house were also fully controlled by researchers. The aim was to assess the performance of Fanger’s thermal comfort model (PMV) in predicting the actual thermal comfort of occupants (AMV) during heating season using two different types of heating emitters. A total of 119 participants between the ages of 19 and 21 years took part in these experiments. AMV of the occupants was determined by conducting surveys whilst PMV was calculated using sensors installed in the living room. Thermal neutral temperatures were calculated and compared for both AMV and PMV indices. It was found that there is a strong and directly proportional relation of both AMV and PMV with the operative temperature in the room. It was also observed that PMV would typically overestimate the neutral temperature compared to the statistically derived neutral temperature which the occupants would consider thermally comfortable

Temperature sensitivity analysis of thermal comfort in a UK residential building

This research focusses on investigating the sensitivity of thermal comfort to temperature in a heated space. Thermal comfort test sessions are conducted in a test house representative of a typical UK house during the winter season. A total of 119 participants took part in the series of tests conducted in the test house’s living room. Operative temperature in the heated space was maintained within the comfortable range recommended by CIBSE for a living room area in a UK house. Two different heating emitters were used for heating during the tests in order to examine their effects on occupant thermal comfort. Conventional radiators supplemented by a gas boiler and an electric fan heater were investigated in the presence and absence of a circulation fan running in the corner of the room. Thermal comfort sensation of occupants was calculated using sensors installed in the living room (Fanger’s Predicted Mean Vote). At the same time the occupants were asked to fill in surveys which were used to record their Actual Mean Vote. From the test sessions conducted it was found that AMV predicted a neutral temperature of 23.5°C whilst PMV predicted a neutral temperature of 24.0°C thus PMV over predicted the occupant’s thermal sensation compared to AMV. For the four heating scenarios it was found that a convective fan heater with a circulation fan causes the smallest temperature gradient (1.0oC) between ankle and head height for a seated occupant according to ISO7730 standards. The highest temperature gradient was measured for fan heater without a circulation fan (7.3oC). Occupants reported to be most uncomfortable if the convector heater without a circulation fan was used

A review of desiccant evaporative cooling systems in hot and humid climates

The use of air conditioning systems has seen considerable growth in the last few decades all over the world, especially in commercial buildings. However, it has shown to have adverse effects on the environment as well as increased energy consumption in buildings. Thus, there has been extensive research to identify alternatives to conventional vapour compression air conditioning systems. Desiccant evaporative cooling (DEC) is one such encouraging system, yet research into its use is still at an initial stage and it needs to be investigated further. In this paper desiccant-based evaporative cooling systems are reviewed, focusing mostly on the solid-based desiccant. The study presents one-stage and two-stage dehumidification, and direct and indirect evaporative cooling for application in hot and humid climates. The review has identified that solid-based desiccant evaporative cooling systems have high potential for use in hot and humid weather. Basic desiccant direct evaporative cooling systems having greater effectiveness in term of output temperature compared to indirect types, but any enhancement in the basic system such as use of an indirect/direct type resulted in a more efficient system. Different system configurations also showed that two-stage systems performed better and displayed lower supply temperatures and humidity ratios compared to the one-stage system

Large eddy simulation of buoyancy-driven natural ventilation-twin-plume flow

This paper presents results of a Large Eddy Simulation (LES) of buoyancy-driven natural ventilation in which two unequal heat sources are used to drive the flow. The aim of this work was to assess the performance of LES in modelling turbulent thermal plumes in a naturally ventilated enclosure and to analyse their interaction with each other. The sub-grid scales of the flow have been resolved by using the Smagorinsky sub-grid scale model. It was found that LES results for the interface height agree well with the theoretical predictions of Linden and Kaye (2006). However, instead of a step change in temperature, as suggested by theory, there exists a more diffuse interface. The merging height of the two plumes and the volume flow rates in the far field merged plume agreed closely with theoretical predictions. Analysis of the pressure isosurfaces demonstrated the interaction phenomena of the two plumes, the coherent structures and the behaviour of the interface

Predicting corrosion rate in chilled HVAC pipe network: Coupon vs linear polarisation resistance method

Research has been undertaken to predict corrosion rate of conveyance pipes used in a Heating Ventilation and Air Conditioning (HVAC) system. This work focuses on comparing the performance of a conventional corrosion predicting technique known as the “coupon method” versus a relatively modern technique called Linear Polarisation Resistance (LPR). An experimental study has been conducted by designing a relatively modern rig that was representative of a typical HVAC system. It was found that both the coupon method and the LPR probe predicted similar corrosion rates with a difference of 8.3% which is insignificant in practical applications. Four highly important in-situ scenarios were simulated for a water temperature range of 12–32 °C, and it was found that an LPR probe with corroded electrodes over-predicted corrosion rate by 20% compared to a clean LPR probe. Also, a probe in use for 2 months over-predicted corrosion by 44% which is a significant error. A pipe network dosed with inhibitor was observed to reduce the rate of corrosion by 50% and 25% for water temperature of 12 °C and 32 °C respectively. It was also determined that one degree (oC) increase in water temperature will increase the corrosion rate by 3.5% on average. It has been identified that both coupon and LPR probes give similar results though both have limitations and continuous monitoring of corrosion is the way to insure a low maintenance HVAC pipe network