Heat and Mass Transfer Enhancement Between a Wet Porous Surface and Airflow
Chapter 1 Overview of energy consumption issues The major areas of concern in energy use include the need to create more efficient conditions in the use of energy in the future by making use of the available energy and avoiding wastage.
There have been energy consumption issues in schools, government agencies and businesses. Energy installers in these institutions need to help their clients reduce energy use by a greater margin to reduce costs and lack of comfort in the use of these sources of energy . As a result, this report provides an understanding of various conditions that exist in energy transfer in buildings and methods that can be used to control heat and mass transfer in buildings. It is significant that challenges are addressed and opportunities embraced because energy production and use in an environmentally benign manner is a platform for improved economic performance and improved quality of life of people in various parts of the world. There have been concerns to ensure that the relationship between energy crises, emissions in buildings and revolutionary ways of design are made to assist in understanding heat and mass transfer in buildings. It has also been observed that at least $18 billion is spent on energy use annually in the US .
The use of energy has also been observed to be higher than production. This results into more pressure on exploitation of natural sources of energy such as oil and natural gas. As a result, there has been the need to get an understanding of ways in which heat and mass transfer takes place in buildings to assist in preventing energy loss. This results into creation of policies that can be useful in managing energy use to prevent wastage. 1.1.
Background There has been growth in demand and use of fossil fuels since industrial revolution that has resulted into high rates of pollution of the air, water as well as deforestation and high concentrations of carbon dioxide in the atmosphere. The use of nuclear energy that was supposed to contribute to a solution to these problems of fossil fuel production and consumption resulted into a greater threat to the environment in terms of wastes and safety . Because all sources of energy have drawbacks that are not regarded beneficial, there has been increased spending on scientific research to determine ways of reducing environmental impact of the sources that are in use, to make these sources more benign and cost-effective, improve efficiency of energy use and determine if energy consumption techniques of humans are causing harm to the planet by irreversibly and harmfully harming the atmosphere . Energy is regarded a major issue. The use of energy is not only the major issues, it is also necessary to determine how energy use can be controlled in buildings by controlling the flow of energy in and out of the building .
It has been necessary to decide the amount of energy used based on a person’s lifestyle, budget and level of comfort. It is also possible to decide the type of energy to be used. For instance, at home a person can use electricity, natural gas, propane, solar, wind or a combination of these. 1.2.Energy Consumption in Building Buildings contribute about 30 to 40 per cent of the total primary energy used globally as shown in figure1.
1. A major account is applied in provision of comfortable indoor climate. The most energy consuming systems are HVAC systems that account for about 10 to 20 per cent of the final energy use in developed countries. For instance, at least 50 per cent of the UK’s CO2 production is contributed by energy used to operate buildings . In regions with cold climatic conditins such as the UK, energy for space heating and hot water contribute to over 60 per cent of the overall energy used in building.
In countries with hot climates such as the Middle East, the growth of energy consumption is attributed to increased use of air conditioning systems in buildings. An example of air conditioning system is one that uses a vapor compressor to remove heat from indoor air to bring thermal comfort . There has been growth in energy consumption in air-conditioning systems in the recent years with electricity generated from fossil fuels being the dominant energy source for air-conditioning systems . This has resulted into increased power loads on electricity grid with consequences of instability in supply of electricity, expensive infrastructure requirements to buildings and more power plants and electrical distribution system, as well as a substantial environmental impact. This has resulted into new challenges that try to investigate the provision of air conditioning for building in relation to their design, manufacturing, management and purposes to improve energy efficiency and contribute to a reduction in electricity consumption. 1.
1.Aim and Objectivesof the Research This project is aimed at devising methods of enhancing convective heat and mass transfer between a wet porous surface and an air flow stream flowing over the surface. Some of the objectives for this study include the following: To investigate the current literature and technologies relating to heat and mass transfer To construct a mathematical model that describes the heat and mass transfer processes To develop and test samples of various porous surfaces with heat and mass transfer enhancements 1.2.Evaporative Cooling and Heat StressHeat stress is perceived by people, animals as well as mechanical and electronic devices especially. In accordance with the guidelines set by the “Occupational Safety Health Administration” (OSHA) and other organizations, a worker who performs his work in environments subject to high temperature, should take frequent breaks in cool environments to avoid the symptoms of stress that the heat causes, or headache, confusion, convulsions, and loss of consciousness .
1.2.1. Working Environment Working environments are considered to be at high temperature when they exceed the thresholds that are the 25 °C for normal jobs and 27 °C for strenuous work. 1.
2.2. Effectiveness in Production For animals and trees, due to excessive heat, adverse effects, such as reduced fertility, reduced milk production for cows and increase the risk of infections and diseases in the warmer months. The stress that causes the heat is a problem that affects mortality in flocks of chickens. The units for cooling by evaporation, such as Port-A-Cool, can lower the temperature of an environment of 10 ° C, then it means that an environment with 32 ° C, in which it is difficult to operate, with such a system comes to have a temperature of about much more favorable than about 21/22 ° C. As temperature increases at the workplace, the efficiency and productivity of the personnel decreases.
1.1.1. People Comfort Indoor thermal comfort is the process of creating a controlled climate in indoor living areas that create comfortable conditions for human beings or animals for proper performance of some induustrial or scientific processes. Modern buildings are intended to provide an acceptable thermal environment for human being .
Human thermal comfort can be referred to as a condition where the mind experiences thermal satisfaction with the thermal surrounding. It is a condition where a person feels thermally neutral and does not know would prefer a higher or lower environmental temperature. Chapter 2 Evaporative Cooling Technology and Working Principle 1.1.The Psychometric Chart This refers to a graphical representation of specific enthalpy, vapor pressure and specific humidity in addition to other characteristics of moist air varying with temperature of the dry bulb while other characteristics such as relative humidity, specific volume and wet bulb temperatures are superimposed on them . There is great significance of physical properties of wet air in the chart that are significant in air conditioning calculations.
In the psychometric charts, the most convenient coordinates are the dry bulb temperature (DBT) plotted along the abscissa and the vapor pressure of water laid out along the ordinate axis.The drawing of saturation curve is accomplished by plotting various saturation points at the corresponding dry bulb temperatures. This is an indication of dry bulb temperatures at 100% relative humidity at various dry bulb temperatures. Since this curve is intersected by constant wet bulb temperatures, it gives dew point as well .One of the most important lines of the chart is dry bulb temperature lines.
These are uniformly distributed lines that are parallel and vertical to the ordinate and extend up to the saturation curve from the abscissa. These lines are spaced at an interval of 5degrees between temperatures of -6 degrees and 45 degrees. The respective values of dry bulb temperatures are indicated on the saturation curve where there is intersection of dry bulb temperatures. The other important lines of the psychometric chart are the specific humidity lines. The symbol for specific humidity is ?. These are horizontal lines that are parallel to the abscissa .
The design of the psychometric chart is done over a range from 0 to 0.030 kg moisture per kg of dry air and they are extended up to the saturation curves. The other important lines of the psychometric chart are the dew point lines. They are horizontal lines that are non-uniformly spaced and extend from the specific humidity axis to the lines of saturation. The points where these lines meet the saturation curve are the dew points. The other important lines of the psychometric chart are the wet bulb temperature lines (WBT).
They are oblique and parallel straight lines that are spaced at non-uniform intervals and they extend up to the saturation curves where twvalues are indicated at their intersection points. The other important lines of the psychometric chart are the enthalpy lines. They are lines obtained by joining similar enthalpy coordinates laid out along the X-Y axes and above saturation line curve. They are inclined straight lines that are parallel to the wet bulb temperature lines . There are also specific volume lines. These are lines that are straight and spaced equally in the psychometric chart and they are drawn up to the saturation curve lines.
The other important lines of the saturation curve are the vapor pressure lines. They are uniformly spaced horizontal lines and are not drawn on the main chart. In addition, there are also relative humidity (RH) lines. These are curved lines that follow saturation curve.