DIMENSIONLESS PHYSICAL-MATHEMATICAL MODELING OF TURBULENT NATURAL CONVECTION

Natural convection heat transfer is present in the most diverse applications of Thermal Engineering, such as in electronic equipment, transmission lines, cooling coils, biological systems, etc. The correct physical-mathematical modeling of this phenomenon is crucial in the applied understanding of its fundamentals and the design of thermal systems and related technologies. Dimensionless analyses can be applied in the study of flows to reduce geometric and experimental dependence and facilitate the modeling process and understanding of the main influence physical parameters; besides being used in creating models and prototypes. This work presents a methodology for dimensionless physical-mathematical modeling of natural convection turbulent flows over isothermal plates, located in an “infinite” open environment. A consolidated dimensionless physical-mathematical model was defined for the studied problem situation. The physical influence of the dimensionless numbers of Grashof, Prandtl, and Turbulent Prandtl was demonstrated. The use of the Theory of Dimensional Analysis and Similarity and its application as a tool and numerical device in the process of building and simplifying CFD simulations were discussed.

Failure mode analysis on compression of lattice structures with internal cooling channels produced by laser powder bed fusion

Conformal cooling coils have been developed during the last decades through the use of additive manufacturing (AM) technologies. The main goal of this study was to analyze how the presence of an internal channel that could act as a conformal cooling coil could affect compressive strength and quasi-elastic gradient of AlSi10Mg lattice structures produced by laser powder bed fusion (LPBF). Three different configurations of samples were tested in compression at 25 °C and 200 °C. The reference structures were body centered cubic (BBC) in the core of the samples with vertical struts along Z (BCCZ) lattices in the outer perimeter, labelled as NC samples. The main novelty consisted in inserting a straight elliptical channel and a 45° elliptical channel inside the BCCZ lattice structures, labelled as SC and 45C samples respectively. All the samples were then tested in as-built (AB) condition, and after two post process heat treatments, commonly used for AlSi10Mg LPBF industrial components, a stress relieving (SR) and a T6 treatment. NC lattice structures AB exhibited an overall fragile fracture and therefore the SC and 45C configuration samples were tested only after thermal treatments. The test at 25 °C showed that all types of samples were characterized by negligible variations in their quasi-elastic gradients and yield strength. On the contrary, the general trend of stress-strain curves was influenced by the presence of the channel and its position. The test at 200 °C showed that NC, SC and 45C samples after SR and T6 treatments exhibited a metal-foam like deformation.

Review on the Influence of Various Types Liquid Desiccants on the Performance of Dehumidification System

Air conditioning is one of the large energy consuming sectors, as it comforts the human in closed environment. Especially, in coastal regions where both temperature and humidity are high, it demands for air conditioning. Usually, in such cases, cooling and dehumidification of outside air is done by cooling coils. Such cooling and dehumidification coils requires relatively large amount of energy. In the era of energy saving technologies, Liquid desiccant system (LDS) has proved to be one of the best alternatives for controlling the humidity, economically and efficiently. Different liquid desiccants are available for dehumidification purpose with their own advantages and disadvantages. This paper summarizes the different liquid desiccants used by researchers in their study and the performances analysis of dehumidification process. An attempt is also made to summarize the characteristics of the different hybrid or composite desiccants developed by researchers in order to improve the desiccant properties for the better dehumidification process.

The Energy Performances of a Ground-to-Air Heat Exchanger: A Comparison Among Köppen Climatic Areas

In this paper the energy performances carried out by an investigation conducted on a Heating, Ventilation and Air Conditioning (HVAC) system, composed by an Air Handling Unit (AHU) for the primary air coupled with a horizontal-pipes Ground-to-Air Heat eXchanger (GAHX) and fan-coil units, for an office building, supposed to be placed in four different cities (Rio de Janeiro, Dubai, Naples, Ottawa) belonging to four different worldwide climatic areas, according to the Köppen climate classification. The investigation is performed by means of a two dimensional numerical model, experimentally validated, of an GAHX solved with finite element method. The results introduced in this paper are carried out by varying the length of the pipes and the air flow velocity; thus, also the number of tubes forming the GAHX is varied since the volumetric flow rate to be provided to the AHU must be kept constant. The energy performances of the above-mentioned HVAC system are analyzed, both in summer and in winter operation modes. Specifically, the reduction of the power of the heating and cooling coils in the AHU due to the pre-treatment of the air operated by the GAHX, the efficiency of the GAHX and the inlet-outlet temperature span are evaluated through a sensitivity analysis. These results lead to say that an GAHX for the geothermal pre-treatment of the air to be introduced into the AHU is energetically very convenient since it leads total thermal power savings in all the investigated cities, that makes this solution competitive. Specifically the best value of power reduction (61.5%) for a 100 m pipe-length GAHX is obtained in Ottawa, a city belonging to continental climate zone. The worst results in terms of power reduction are registered in Rio de Janeiro, a city belonging to the tropical or equatorial climates: this reduction, for a 100 m pipe-length ground to air heat exchanger, is 23.9%.

Simulation of a Solar-Assisted Air-Conditioning System Applied to a Remote School

In this work, we present an absorption cooling system with 35 kW capacity driven by solar thermal energy, installed in the school of Puertecitos, Mexico, an off-grid community with a high level of social marginalization. The cooling system provides thermal comfort to the school’s classrooms through four 8.75-kW cooling coils, while a 110-m2 field of evacuated tube solar collectors delivers the thermal energy needed to activate the cooling machine. The characteristics of the equipment installed in the school were used for simulation and operative analysis of the system under the influence of typical factors of an isolated coastal community, such as the influence of climate, thermal load, and water consumption in the cooling tower, among others. The aim of this simulation study was to determine the best operating conditions prior to system start-up, to establish the requirements for external heating and cooling services, and to quantify the freshwater requirements for the proper functioning of the system. The results show that, with the simulated strategies implemented, with a maximum load operation, the system can maintain thermal comfort in the classrooms for five days of classes. This is feasible as long as weekends are dedicated to raising the water temperature in the thermal storage tank. As the total capacity of the system is distributed in the four cooling coils, it is possible to control the cooling demand in order to extend the operation periods. Utilizing 75% or less of the cooling capacity, the system can operate continuously, taking advantage of stored energy. The cooling tower requires about 750 kg of water per day, which becomes critical given the scarcity of this resource in the community.

Theoretical Parametric Study of Wrap-Around Heat Pipe (WAHP) in Air Conditioning Systems

Warp-around heat pipes (WAHP) belong to a special class of recuperative heat exchangers that transfer heat from inlet to outlet locations via thermal gradient, without using any energy. In the present work, effects of various mechanical parameters on the performance of a WAHP dehumidifier system that are based on the underlying principles of heat and mass conservation are presented primarily from a theoretical point of view. A simplified methodology pertaining to wet cooling coils is applied here for defining the case of moisture condensation during precooling process at WAHP evaporator. Inlet air temperature, inlet humidity ratio, air mass flow rate, dehumidifier outlet temperature and effectiveness are the main operational parameters considered in this study. On the other hand, comparative performance study of the WAHP system is done through an other set of thermodynamic parameters like the supply air temperature, supply humidity ratio, specific coil load and recovered enthalpy. The subtle variations in these factors against the operational parameters not only help in stipulating functional characteristics of the WAHP but also allow HVAC designers to make informed decisions for system design and performance without relying entirely on manufacturer’s equipment data.

Aerosolization of Aspergillus niger spores from colonies on different positions of a circular tube

Subject to the blowing air, fungal spores can be aerosolized from the colonies growing on cooling coils. The blowing air may accelerate and decelerate when passing a coil. The caused spore detachment may be different, when the fungal colonies grow on different positions of the coil. This study investigated the aerosolization of Aspergillus niger spores from a colony on a circular tube in a wind tunnel. The colony was first cultured in the carved groove along the longitudinal tube, and then the spores were aerosolized by a gradual increase of the blowing air speed. The grown colony on four different positions of the coil surface were blown for aerosolization. In addition, the airflow surrounding the circular tube was numerically solved to estimate the drag force for aerosolization of the spores. The results revealed that the collected airborne spores when the colonies were located tangentially to the upcoming airflow were six to eight times of those when the colonies were located parallel with the upcoming airflow for both the 4-day-old and 10-day-old colonies. The local air speeds in different positions of the tube were highly different from the inlet air speed of the wind tunnel. Such difference should be accounted for when estimating the drag force to aerosolize the fungal spores on the coils.

Environmental Modelling and Simulation for Naval Ships

The existing HVAC system on board Landing Platform Docks (LPD)s, has been designed to operate in mild tropical environmental conditions such as the Mediterranean. This means that when the ship operates in extreme hot climates such as the Arabian Gulf and the Red sea region, the HVAC system is over loaded and cannot maintain the required design conditions. This results in uncomfortable and difficult working conditions for the ship’s occupants, and causes problems with equipment overheating, and overloading of cooling plants. In order to assess options for improving the HVAC system, an environmental model that is capable of dynamically simulating the environment inside the ship is created. The model takes into account weather data based on ship location and time, ship structure and insulation, sun location and shading effects, internal heat gains from equipment and people, and the performance of the HVAC systems; which includes chillers, cooling coils, heaters, controllers, etc. The environmental model is used to assess options for improving the HVAC system to accommodate extreme tropical conditions. The model also forms an environmental baseline for the ship that can be used for assessing the impact of any change in the operational profile of the ship.