The Armfield HT10XC is a computer controlled service unit, which can be used in conjunction with a range of small-scale accessories for a wide range of demonstrations into the modes of heat transfer. The factors that affect heat transfer can be investigated and some of the practical problems associated with the transfer of heat can be clearly demonstrated. The heat transfer accessories may be individually connected to the HT10XC service unit, which provides the necessary electrical supplies and measurement facilities for investigation and comparison of the different heat transfer characteristics. A specific feature of the HT10XC is that it corporates the facilities and safety features to enable the accessories to be remotely controlled from an external computer, where this is appropriate. It will shut down safely in the event of a communications failure. With suitable (user provided) software, the equipment operated remotely, for instance over an intranet or even over the internet. All the facilities can also be accessed locally using the front panel controls and display.
The service unit is housed in a robust steel enclosure and designed for use on a bench or table. It provides control outputs to the accessories, and instrumentation inputs from the accessories.
Inputs and Instrumentation:
In manual mode, the outputs listed above are under control of potentiometers on the front panel of the unit. In remote mode the outputs to the accessories are controlled by the computer. A ‘Watchdog’ system is implemented in remote mode to ensure operator and equipment safety in event of a computer or communications failure.
In both modes the signals from the accessory can be shown on the front panel displays. Selector switches are used to select the chosen signal onto one of the two displays. These signals are also available on the USB interface for datalogging on the computer (even if the computer is not controlling the equipment).
The Armfield Linear Heat Conduction accessory has been designed to demonstrate the application of the Fourier Rate equation to simple steady-state conduction in one dimension.
The units can be configured as a simple plane wall of uniform material and constant cross sectional area or composite plane walls with different materials or changes in cross sectional area to allow the principles of heat flow by linear conduction to be investigated. Measurement of the heat flow and temperature gradient allows the thermal conductivity of the material to be calculated. The design allows the conductivity of thin samples of insulating material to be determined.
On the HT11C the heater power and the cooling water flow rate are controlled via the HT10XC, either from the front panel or from the computer software. On the HT11 these are controlled manually.
The Armfield Radial Heat Conduction accessories have been designed to demonstrate the application of the Fourier rate equation to simple steady-state conduction radially through the wall of a tube.
The arrangement, using a solid metal disk with temperature measurements at different radii and heat flow radially outward from the centre to the periphery, enables the temperature distribution and flow of heat by radial conduction to be investigated. On the HT12C the heater power and the cooling water flow rate are controlled via the HT10XC, either from the front panel or from the computer software. On the HT12 these are controlled manually.
This Armfield accessory has been designed to demonstrate the laws of radiant heat transfer and radiant heat exchange using light radiation to complement the heat demonstrations where the use of thermal radiation would be impractical. The equipment supplied comprises an arrangement of energy sources, measuring instruments, aperture plates, filter plates and target plates, which are mounted on a linear track, in different combinations, to suit the particular laboratory teaching exercise chosen.
A hot surface loses heat (heat is transferred) to its surroundings by the combined modes of convection and radiation. In practice these modes are difficult to isolate, so an analysis of the combined effects at varying surface temperature and air velocity over the surface provides a meaningful teaching exercise.
The heated surface studied is a horizontal cylinder, which can be operated in free convection or forced convection when located in the stream of moving air. Measurement of the surface temperature of the uniformly heated cylinder and the electrical power supplied to it enables the combined effects of radiation and convection to be compared with theoretical values. The dominance of convection at lower surface temperatures and the dominance of radiation at higher surface temperatures can be demonstrated as can the increase in heat transfer due to forced convection.
On the HT14C, the heater power and the air flow are controlled via the HT10XC, either from the front panel, or from the computer software. On HT14 these are controlled manually.
A long horizontal rod, which is heated at one end, provides an extended surface (pin) for heat transfer measurements. Thermocouples at regular intervals along the rod allow the surface temperature profile to be measured. By making the diameter of the rod small in relation to its length, thermal conduction along the rod can be assumed to be one-dimensional and heat loss from the tip can be ignored. The measurements obtained can be compared with a theoretical analysis of thermal conduction along the bar combined with heat loss (heat transferred) to the surroundings by the modes of free convection and radiation simultaneously.
Radiative heat transfer between a thermometer and its surroundings may significantly affect the temperature reading obtained from the thermometer, especially when the temperature of a gas is to be measured while the thermometer ‘sees’ surrounding surfaces at a higher or lower temperature than the gas. The error in the reading from the thermometer is also affected by other factors such as the gas velocity over the thermometer, the physical size of the thermometer and the emissivity of the thermometer body.
In this equipment a group of thermocouples are used to measure the temperature of a stream of air, at ambient temperature, passing through the centre of a duct while the wall of the duct is elevated in temperature to subject the thermocouples to a source of thermal radiation. Each thermocouple gains heat by radiation from the heated wall and loses heat by convection to the air stream and conduction along the wire. The net result is an increase in the temperature of the thermocouple above the temperature of the air stream which it is supposed to measure. The result is an error in the reading from the thermocouple. A radiation shield can be positioned in the duct to show the effect of screening the thermocouples from thermal radiation from the duct wall.
On the HT16C the heater power, the air flow rate and the position of the radiation shield can all be controlled via the HT10XC, either from the front panel controls or from the software. On HT16, these parameters are adjusted manually.
Analytical solutions are available for temperature distribution and heat flow as a function of time and position for simple solid shapes which are suddenly subjected to convection with a fluid at a constant temperature. Simple shapes are provided together with appropriate classical transient-temperature/heat flow charts which allow a fast analysis of the response from actual transient measurements. Each shape is allowed to stabilise at room temperature then suddenly immersed in a bath of hot water at a steady temperature. Monitoring of the temperature at the centre of the shape allows analysis of heat flow using the appropriate transient-temperature/heat flow charts provided.
An independent thermocouple mounted alongside the shape indicates the temperature of the water adjacent to the shape and provides an accurate datum for measurement of the time since immersion in the hot water.
Based on a Peltier device, the Armfield HT18C Thermo-electric Heat Pump demonstrates how electrical power can be used to extract heat from a cool surface and transfer it to a hot surface.
This effect is becoming widely used for point cooling (eg of semiconductor devices) and small-scale volumetric cooling. The HT18C is designed for use with the Armfield HT10XC Heat Transfer Teaching Equipment.
The Armfield Free and Forced Convection unit has been specifically designed to demonstrate the phenomena of natural (free) and forced convection. Temperature profiles and heat flux over three different heat transfer surfaces can be easily studied.
The HT19 is designed for use with the Armfield HT10XC Heat Transfer Teaching Equipment.
The Armfield Conductivity of Liquids and Gases unit has been specifically designed to enable students to measure and compare the thermal conductivities of various liquids and gases. It’s designed to facilitate quick and effective cleaning and to minimise thermal losses.
The HT20 / HT20C is backwards compatible with the HT10XC, so if you already own an HT10XC, you can easily expand the teaching potential with the addition of this accessory.