Masahito Oguma

Development of a method to estimate profiles of equivalent absorption coefficient for gray analysis

Using inverse radiative property value analysis, profiles of gray equivalent absorption coefficient are estimated with results similar to non-gray analysis when the coefficients are used in the corresponding gray analysis. For the inverse analysis, the results of non-gray analysis are used as the input. An approximate method is proposed for the estimation of the profiles of equivalent absorption coefficients without previously knowing the results of non-gray analysis. The profiles of the equivalent absorption coefficients obtained by the new method are compared with that estimated by the inverse analysis for two-dimensional model furnaces. The new method is shown to be useful in estimating the profile of absorption coefficients for gray analysis.

The Effects of Groundwater Flow on Vertical-Borehole Ground Source Heat Pump Systems

Heat advection by groundwater flow is known to improve the performance of ground heat exchangers (GHEs), but the effect of groundwater advection on performance is not yet fully understood. This study examined how parameters related to groundwater flow, such as aquifer thickness, porosity, lithology, and groundwater flow velocity, affect the performance of a borehole GHE. Under the thin-aquifer condition (10 m, or 10% of the entire GHE length in this study), groundwater flow velocity had the greatest effect on heat flux. With a groundwater flow velocity of at least 10−4 m/s through a low-porosity aquifer filled with gravel with high thermal conductivity, the heat flux of a GHE was as much as 60% higher than that of a non-aquifer GHE. If the aquifer is as thick as 50 m (50% of the entire GHE length), the high thermal conductivity of gravel doubled the heat flux of the GHE with a groundwater flow velocity of at least 10−5 m/s. Thus, not only groundwater flow velocity, but also aquifer thickness and thermal conductivity were important factors. However, groundwater seldom flows at such high velocities, and porosity, gravel size, and aquifer thickness vary regionally. Thus, in the design of ground source heat pump systems, it is not appropriate to assume a large groundwater effect.Copyright

Numerical Solution of a Ground Source Heat Pump System Using Foundation Piles

A ground source heat pump (GSHP) system has higher cooling and heating performances than an air source heat pump system, so the GSHP system has attracted attention in the cold regions. Particularly after the 2011 earthquake off the Pacific coast of Tohoku, which damaged nuclear power plants, the GSHP utilizing renewable energy, has become attractive in heating system in the Tohoku region. However, it is necessary to install wells to collect ground source heat, and so, together with the cost of the heat pump itself, the installation cost is a barrier to widespread adoption of this technology in Japan.On the other hand, due to poor subsoil, foundation piles are often required in the construction of buildings in Japan. By using foundation piles as heat exchangers, which are commonly used in residential construction, the cost of using GSHP systems in houses may be reduced. However, since the placement of the piles depends on the floor layout of the residence, that is arbitrary sequence. Moreover, an arbitrary floor layout requires a complicated multi-dimensional numerical analysis to design the GSHP, and the analysis is burdensome for general designers. Therefore, the use of the model unit of the two-dimensional cylindrical heat exchange well is proposed. The use of this model, which includes an unused volume of soil, reduces the analysis burden for general design tools.On experience, the arrangement rate is 4 m2 per pile, and the well separation is narrow. And the foundation piles will form a group. So thermal interference between heat exchanger wells might be working hard. In addition, the foundation piles are very short, compared to the traditional borehole depth of 50 to 100 m. Therefore, the thermal performance of the well may degrade compared to that of the traditional GSHP system, although the initial costs are less. Therefore, we examined a GSHP system with heat exchanger piles by using the heat exchange well unit model for residences in the Tohoku region.As a result, the two-dimensional analysis and more is needed to predict the thermal performances of the heat exchanger piles so that the effect of the pitch of the heat exchange wells on the heat transfer in the axial direction will be large. In cold climates, the heating demand is large and, in the summer, the amount of waste heat to the ground is small, so a large amount of heat penetration into the ground from the atmospheric air is important for continuous GSHP operation in the Tohoku region, and in Fukushima Prefecture in particular.Copyright

Thermal design of a hydrogen heater for an air turboramjet engine

Abstract A compact heat exchanger has been designed for an air turboramjet engine proposed for future spacecraft. Hydrogen fuel is heated by the combustion gas of the engine at the heat exchanger. The heat exchanger, called a hydrogen heater when used in an aircraft, has to be compact so that when it is set in an engine duct to enhance heat transfer it will cause a small increase in the pressure loss of the combustion gas. To endure the vibration caused by combustion gas turbulence and the high pressure of hydrogen fuel flowing in the heat exchanger, inline tangential tube banks are adapted. An optimum design point of the heat exchanger is shown that is calculated by use of results of fundamental experiments on the heat transfer of inline tangential tubes. A small heat exchanger is developed and loaded into an ATREX500 type engine, a sea-level static air turboramjet engine used to check the predicted thermal performance of the heat exchanger. Experimental results almost match the engine system requirements and show the possibility of developing a flight-model heat exchanger.

The likelihood evaluation of the heat exchanger unit model design method in the ground source heat pump system utilizing heat exchanger piles

A ground source heat pump system (GSHP) utilizing the foundation piles as heat exchangers is expected to reduce the initial cost compared with the borehole type of the conventional system because the heat exchanger piles is not needed to install anew. However, the arrangement of the piles is depend on the floor layout of a residence. Therefore, the thermal performance prediction of heat exchange piles has to strictly conduct three-dimensional analysis for every residence. Conducting such analysis, it is a heavy burden for a designer to perform performance prediction of the GSHP with satisfaction of the annual heating and cooling demand. Then, the two- dimensional heat unit model was proposed in order to ease a designers burden. This model has the characteristic of originally not using the soil portion where the ground heat source is available. And if it is seemed to be the design likelihood, it makes the designers load reduce. The introductory possibility of the GSHP, which uses the foundation pile in the semi-cold district of our country, is shown by the numerical analysis using this model. It is necessary to show clearly how much the analytical results calculated by use of this model has likelihood. Then, as compared with the higher-dimensional analysis results, the same heating demand conditions are employed at the thermal prediction. The rectangular and triangle arrangements of piles are inquired for the evaluations. The finite difference method is employed for the numerical analysis. It is shown that the evaluation using the proposed model has an average of 10 - 50% of likelihood compared with the higher dimensional analysis evaluation when single layer of piles surrounds the center piles. In the case of the arrangements in which multi layers of piles surround the center piles, the proposed thermal unit model leads the same analytical results by use of higher dimensional coordinates model.

Evaluation of Measurement Accuracy of Underground Thermometers Using Steel-Pipe Piles

For ground source heat utilization systems, pile heat exchangers are sometimes used. In order for these systems to achieve high performance, control of the system dynamics is important, and the underground temperature must be known. Typically, underground temperature is measured using a thermometer in a borehole. However, in the case of pile heat exchangers, a different method is required, making the system expensive to set up. To overcome this problem, the installation of underground thermometers in the heat exchanger piles themselves is proposed in the present study. The proposed thermometer system consists of thermocouples packed in grout such as silica sand within the piles. However, there is a possibility of measurement errors due to vertical thermal conduction in the steel pipes, and it is important to estimate the measurement accuracy before the development of this system. In the present study, the measurement accuracy is estimated using numerical simulations and then confirmed experimentally. The underground temperature profiles inside and outside the pile are compared. The results indicate that the proposed system offers sufficient accuracy for application to pile heat exchangers.

Optimum Heating Pattern of a Ground Source Heat Reference Map

A ground source heat reference map (GSHRM) shows the minimum necessary thermal performance of the ground heat exchanger (GHE) of a ground source heat pump (GSHP) system. Thermal performance depends on thermal properties of the ground, the ground temperature profile, heat advection by groundwater flow, and the GHE operating pattern. This study modeled optimum heating and cooling modes for a GSHRM. First, continuous and intermittent operation modes were compared, and a standard operation time was defined. In a standard household GSHP system, the quantity of heat transferred from the ground depends on household energy demand, which is relatively constant. Once the demand is known, an operation mode is selected that can meet it. Continuous operation increased the total amount of heat exchanged over a period of time but lowered the heat flux at the GHE, whereas intermittent operation with relatively long stopped periods decreased the total amount of heat but did not greatly decrease the heat flux at the GHE. Second, energy-saving efficiency and cost factors were compared among intermittent operation modes. Operation costs consist of the electrical energy supplied to the heat and circulation pumps. At a given operation time, the energy supplied to the heat pump depends on its coefficient of performance (COP), whereas that supplied to the circulation pump depends on its pressure loss, hence on the GHE length. A long GHE has a higher initial cost. Thus, the optimum heating pattern must consider the configuration of the GSHP system, including energy-saving efficiency and cost factors.Copyright

Research & development of the ground source heat reference map: The effect of the heat collecting pattern and the ground water flow

Ground source heat reference map (GSHRM) for designing the ground source heat pump system is proposed. By use of the map, designers of the ground source heat pump system are able to predict the thermal performance of the ground heat exchangers (GHEs) without the pre-evaluation test of the GHEs. In this map, the advection of the heat transfer due to the ground water flow is not expected, and is set to be the design margin. Also, the ground temperature is assumed to be the average atmospheric temperature. So, the map will show the minimum thermal performance of the GHEs with these conditions. To decide the thermal performance, the operation pattern of GHEs is necessary, and this pattern should not be any. In this paper, it is shown that the maximum cooling or heating operation pattern is existed, and this pattern is proposed to be the standard operation pattern. Also, the advection by groundwater flow is evaluated.

Prediction of the Heat Transfer and Flow Characteristics in Rotary Combustor

A numerical analysis model is developed to evaluate heat transfer and fluid flow characteristics of a through-flow-type-rotary system. The numerical domain is divided into three parts, waste layer, gas phase above the layer and rotary grate, and these are modeled, respectively. The analytical results of CO concentration in gas at outlet, peak temperature of rotary grate and heat-recovering rate correspond with the experimental those, which shows validity of the numerical model. The maximum temperature of the layer, heat recovering rate and NO concentration in gas at outlet rise in proportion to O2 concentration in air into combustion zone, and unburned carbon in combustion residue and CO concentration at outlet and decrease at a fixed rate. In case that flue gas is recirculated from the top of incinerator into combustion zone, NO concentration in gas at the outlet decreases in proportion to the flow rate of recirculating gas.Copyright

Development of Estimation Method of Profiles of Equivalent Absorption Coefficient for Gray Analysis Giving Similar Results with Non-gray Analysis.

Using inverse radiative property value analysis, profiles of gray equivalent absorption coefficient are estimated which give results similar to the ones of nongray analysis when the coefficients are used in the corresponding gary analysis. For the inverse analysis, the results of nongray analysis are used as the input. A new approximate method is proposed for the estimation of the profiles of equivalent absorption coefficient without knowing tha results of nongray analysis previously. The profiles of the equivalent absorption coefficient obtained by the new method is compared with the ones estimated by the inverse analysis for six practical cases. And the new method is shown to be useful for estimating the profile of absorption coefficient for gray analysis in most cases.