Benny Bøhm

Aggregated dynamic simulation model of district heating networks

Abstract The dynamic properties of district heating (DH) networks include water flow and propagation of heat from production plants to consumers. Mathematical models of such networks can be applied, either for general understanding of DH systems, or in combination with production planning and optimization. One type of mathematical model involves a full physical modeling of the network, taking into account individual pipes, dimensions, material properties etc. Such full models tend to be computationally intensive when applied in network simulations, which can be a problem when considering large DH systems. In the current paper, a method is presented in which a fully described model of a DH network is replaced by a simplified one, with the purpose of reducing simulation time. This simplified model is generated by gradually reducing the topological complexity of the original network. The method is validated by applying it on a real case study, in which a network with over 1000 pipes is reduced to less than 10 pipes. The results show that such relatively simple networks can maintain most of the dynamic characteristics of the original networks.

On transient heat losses from buried district heating pipes

The theory of steady-state heat loss determination from buried heating pipes has been reviewed as well as previous approaches to treat transient heat losses in the case of constant water temperatures. A new method has been developed to find an undisturbed ground temperature by which the transient heat loss can be calculated by the use of the steady-state heat loss equations. By numerical simulations, as well as by experiment, the position of this undisturbed ground temperature has been found. The position of this ground temperature is closer to the ground surface in the case of uninsulated pipes—or pipes with the insulation in poor condition—than in the case of insulated pipes. For pre-insulated pipes the position corresponds approximately to the top of the pipe casing. Copyright

Evaluation of approaches for modeling temperature wave propagation in district heating pipelines

The limitations of a pseudo-transient approach for modeling temperature wave propagation in district heating pipes were investigated by comparing numerical predictions with experimental data. The performance of two approaches, namely a pseudo-transient approach implemented in the finite element code ANSYS and a node method, was examined for a low turbulent Reynolds number regime and small velocity fluctuations. Both approaches are found to have limitations in predicting the temperature response time and predicting the peak values of the temperature wave, which is further hampered by the fact that the fluid is represented as an ideal fluid. The approaches failed to adequately predict the temperature wave propagation in the case of rapid inlet temperature changes. The overall conclusion from this case study was that in order to improve the prediction of the transient temperature, attention has to be given to the detailed modeling of the turbulent flow characteristics.

Nonconventional fully developed polyethylene and wood compartment fires

Abstract Seventeen experimental fires with polyethylene and wood fuel were carried out in a full-scale compartment. The combustion air was supplied by a fan and the special feature of the compartment was the use of a slit which enabled the vertical position of the neutral plane to be well defined for outside observation. Large vertical and horizontal gradients in the gas temperature were found. In the polyethylene fires the highest mean compartment temperature measured was 1200°C. In the fully developed period all fires were ventilation controlled. Simulations of the experiments were carried out by means of a computer program. A general design model for polyethylene fires is proposed. This model is compared with a similar model for wood fires, and the models are applied to structural design. For wood structures and unprotected or lightly protected steel members the polyethylene fire is the more severe.

Dynamic Temperature Simulation in District Heating Systems in Denmark Regarding Pronounced Transient Behaviour

A dynamic performance of district heating systems was analysed with an emphasis on temperature profile distortion throughout a heating system network. Therefore, a modelling approach (the so-called node method) developed at the Technical University of Denmark was applied. For comparison purposes, commercial TERMIS software was also used in this work. Typical supply conditions were investigated in the considered district heating systems in Denmark. Large and sudden temperature changes in supply temperature were depicted in the district heating system in Madumvej while the pronounced transient supply conditions of temperature wave were exhibited in the district heating system in Naestved. Time-dependent consumer data from the district heating systems were applied to compare the results obtained from modelling approaches. Based on the analysis of temperature wave propagation through the network, it was noted that temperature wave spread unevenly on different locations of the network.

The development of a small-scale fire compartment in order to determine thermal exposure inside and outside buildings

Abstract In order to investigate some aspects of the thermal exposure in building fires, a series of measurements were taken in a fire compartment with the dimensions 0.8 m × 0.8 m × 1.2 m . The fuel was propane and the fires were fully developed. The heat release rate inside the compartment as well as in the external flame outside the opening were measured. Temperatures and heat fluxes inside and outside the compartment were also measured.

Calculation of heat transfer in fire test furnaces with specific interest in the exposure of steel structures. A technical note

Abstract The equations for radiactive transfer in a compartment are outlined for a uniform gray gas. The incident radiation towards a test structure is related to the incident radiation from a black gas by the parameter φ. φ increases with time as the compartment walls are heated. φ values are plotted in a figure, which makes manual calculation possible. In computer programmes time dependence of gas- and wall surface emissivities can be accounted for. In general the ϵ res approach is incorrect. For steel structures in a uniform test furnace a comparison is made between the two approaches, and it is shown that the ϵ res approach overestimates the thermal exposure. The “overfiring” of test furnaces due to the fact that the ISO curve is specified for a thermocouple (and not the true gas temperature) partly compensates this error.

Evaluation of the dynamic performance of a hot water tank with built-in heating coil

Hot water tanks with a built-in water-heating coil are commonly used in district heating house stations in Denmark for domestic hot water (DHW) production and storage. In this study, an evaluation of the dynamic performance of a hot water tank with built-in heating coil is carried out by applying a dynamic simulation programme which has been made previously, based on a simple dynamic model developed by the authors. System evaluation of the way in which system parameters, such as control valve size, heat loss coefficient of the DHW circulation pipe, position of the temperature sensor (for DHW temperature control) and fouling of the heating coil, affect the domestic hot water capacity and the average district heating water cooling for a given hot water tank is presented and discussed in this paper. The evaluation results show the importance of the correct design of the control valve size, the reduction of heat loss from DHW circulation pipes, the careful adjustment of temperature sensor position and temperature sensor set-point, and the reduction of the heat coil fouling growth rate in order to operate the hot water tank in an efficient way and to achieve significant cooling of the district heating water.

On the calibration and application of heat flux sensors on buried district heating pipes

A heat flux sensor (HFS) can be used to measure the heat loss from buried district heating (DH) pipes if the HFS is calibrated in conditions resembling the actual condition of use, i.e. not in one-dimensional conditions. Owing to the fact that the thermal conductivity of the HFS often differs from the thermal conductivity of the surrounding media, the heat flux through the HFS will differ from the true heat flux. Consequently the development of compensated HFSs is discussed. The influence of how the HFS is fixed to the pipe casing is discussed as well as the influence of the casing diameter and the soil thermal conductivity on the calibration factor. The long-term stability of HFSs is discussed with reference to measurements on a DH transmission line, which were carried out over a period of more than one year. Copyright