Briefly describe and address the problem or issue:

            The main issue concerned when it comes to storing frozen food, like ice kacang, is to maintain the temperature of the food so as to prevent it from melting. When these frozen food items are stored in normal plastic containers, heat enters by several ways, mainly conduction, convection and radiation, causing the frozen food items to gain heat and begin to melt.

          Frozen food items melt when they gain heat. These substances remain frozen only when its temperature is kept at or below its ice point. For example, in the ice of ice kacang, the temperature of the ice is to be kept at 0ºC or lower. This is so as the ice point, or the melting point of the ice is 0ºC. Hence, to keep its molecules vibrating slowly in a fixed position, a low temperature is required so as to not break the bonds holding these molecules together.

          Placing the ice kacang in a normal container can cause heat to enter through conduction, convection and radiation.

          Firstly, most of the heat gained by the ice kacang is through conduction. Conduction is the transfer of heat energy through a material medium, solid, liquid or gas, by the passing on of increased atomic and/or molecular vibrations from the hot end to the cold end. In the case of a normal plastic container, the following process occurs. When one end of the solid is heated, the particles at this end gain energy and vibrate faster. The neighbouring particles are jostled and made to vibrate more vigorously. This would then go on along the solid. After some time, the increased vibration is spread from the hot end to the cold end and eventually to the ice kacang. Hence, this would increase the temperature of the ice kacang, causing it to melt.

          Secondly, some heat is also gained by the ice kacang through convection. Convection is the transfer of heat energy by the bulk movement of fluid, liquid or gas, due to changes in density. In the container storing the ice kacang, there would be some air available. Hence, convection of heat is one of the factors leading to the melting of the ice kacang.

       

 Lastly, heat is also lost through radiation.

Radiation is the transfer of heat energy by the emission and absorption of infra-red rays. When an object radiates heat, it is said to emit radiation. The rate at which a body radiates heat would depend on its temperature and the nature of its surfaces. For example, a dull surface is a better emitter or radiator while a polished, shiny surface is a good reflector of radiation. Since the containers used to contain ice kacang are usually smooth and light coloured, they prove to be a very good radiator of heat.

          Therefore, the issue in storing ice kacang would be the TYPE, DESIGN AND TEXTUREof the containers that would be designed.

Processes of thermal heat energy:

(i) Conduction

Heat gets transferred from one atom to the other.

Heat conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their heat energy to these neighboring particles. Heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from one atom to another. 

Conduction is the most significant means of heat transfer within a solid or between solid objects in thermal contact. Gases and liquids are less conductive, hence convection is more dominant in gases and liquids. 

Thermal contact conductance is the study of heat conduction between solid bodies in contact. Hence, due to heat conduction, materials that can easily conduct heat tend to spread heat faster, thus materials that are poor conductors of heat should be used.

(ii) Convection

Convection is the transfer of heat from one place to another by the movement of fluids. It is a process that is essentially the transfer of heat via mass transfer. Bulk motion of fluid enhances heat transfer in many physical situations, such as, between a solid surface and the fluid. Convection is usually the dominant form of heat transfer in liquids and gases.

Convection occurs when bulk fluid motion, for example, steams and currents are caused by buoyancy forces that result from density variations due to variations of temperature in the fluid. Forced convection is a term used when the streams and currents in the fluid are induced by external means—such as fans, stirrers, and pumps—creating an artificially induced convection current. 

Thus, convection accounts for how heat travels within liquids and gases and thus, some substances, such as water, are poor conductors of heat and air, is a poor conductor of heat as well.

(iii) Radiation

Radiation is often concentrated to penetrate items.

Thermal radiation is energy emitted by matter as electromagnetic waves due to the pool of thermal energy that all matter possesses that has a temperature above absolute zero. Thermal radiation still occurs without the presence of matter through vacuum.

Thermal radiation is a direct result of the random movements of atoms and molecules in matter. Since these atoms and molecules are composed of charged particles (protons and electrons), their movement results in the emission of electromagnetic radiation, which carries energy away from the surface.

Unlike conductive and convective forms of heat transfer, thermal radiation can be concentrated in a small spot by using reflecting mirrors, which is exploited in concentrating solar power generation.

Hence, radiation allows heat to be transferred no matter what the circumstances are. It is often used in thermal flasks to ensure the substance in it will not lose or gain heat easily.

(iv) Advection

Advection is a way of transportation. It often refers to horizontal transportation of heat and humidity.

The way the silt is advected (transported) downstream in the rivers, in the same way everything can be advected in any fluid. Advection is also important part of hydrological cycle; in formation of oragraphic clouds and the precipitaton of water from clouds.

For heat, by transferring matter, energy, including thermal energy, is moved by the physical transfer of a hot or cold object from one place to another. Placing hot water in a bottle and heating a bed are examples of advection. Another practical example is thermal hydraulics.

About the box we created:

Requirements: 

Poor heat conductor, little air in container, light and polished surface (not dull)

 

The box we created is made of styrofoam and it is wrapped with aluminum. I

nside the styrofoam box is another layer of styrofoam.

The second layer of styrofoam that is sealed with tape creates an artificial vacuum. Dead air is a very good insulator of heat because its molecules have a lot of spaces between them. The large amount of spaces between the molecules does not facilitate the conduction of heat which requires molecules to be relatively close to each other so that when one molecule gains enough heat and kinetic energy to vibrate, this heat energy can then be transferred to another molecule. In addition, styrofoam is a poor conductor of heat so the process of conduction is also further restricted. Therefore, these will reduce the heat gain of the container holding the ice kacang by reducing the conduction of heat in the container.

Secondly, Styrofoam can trap air in its foam pockets. This prevents some air from moving about freely throughout the container. Since there is more still air (the one trapped in the pockets) than freely moving air, convection of the air (bulk movement of the air due to changes in density) is not very efficient. This makes Styrofoam a good insulator of heat. Since convection cannot occur as well in a Styrofoam container than in a plastic container, we selected Styrofoam so that it can reduce the heat gained inside the container by convection.

Styrofoam is used because it is white in colour and is able to reflect heat more effectively. This albedo effect allows the container to reflect some of the incoming heat from its surroundings, preventing a large amount of heat from penetrating into the container. Black surfaces absorb a lot of heat, therefore we chose to use a white surface which will absorb lesser heat so the container holding the ice kacang would not gain too much heat. Furthermore, we wrapped aluminum around the Styrofoam. Aluminum has a shiny surface. Unlike dull surfaces, the shiny surface of aluminum reflects heat better. In fact, dull surfaces radiate heat much more than shiny surfaces. These prevent the ice kacang from melting easily due to radiation.

Bibliography: 

http://en.wikipedia.org/wiki/Heat_transfer#Conduction

http://arcticstudies.pbworks.com/w/page/13623277/Advection
http://www.google.com

Done by:  Eunice Lim (17), Michelle Ko (21), Beatrice Ng (22), Theresa Phua (24)