•The outdoor air relative humidity also has an impact on the threshold temperature of frost accumulation.•Under certain conditions, higher efficiency of heat recovery may be safer in terms of frost ...risk.•The largest ‘frost’ accumulation area is formed for the middle values of the return air relative humidity (in analyzed case RH2i=38.5 %).
In this paper, a process of frost accumulation inside the non-hygroscopic rotary heat exchanger depending on operating conditions is investigated. The study was based on the results of the numerical simulations conducted in the original computer program. The analysis considers both the threshold conditions of frost accumulation and the conditions of frost layer growth. For this purpose, three indicators related to frost accumulation were distinguished: a frost accumulation limit temperature, a surface of the matrix covered with the accumulated frost layer, a maximum thickness of the accumulated frost layer. The analyzes assessed the influence of operating parameters on these indicators, focusing on the return air relative humidity. It was revealed that the highest values of this parameter cause the greater thickness of the frost layer. However, the largest ‘frost’ accumulation area was created in the range of average return air relative humidity values – in the analyzed cases it was RH2i=38.5 %. The paper also includes a detailed analysis of the threshold temperature of frost accumulation. These limits of outdoor air temperature were determined for a wide range of operating parameters’ values: both airstream relative humidity and nominal temperature effectiveness. Additionally, eight probable configurations of active heat and mass transfer areas within rotary heat exchanger channels were revealed, which are related to the initiation of frost accumulation. Consequently, the locations of the initial ‘frost’ accumulation area were predicted. It was also found that the greatest impact on the frost accumulation threshold temperature has the return air relative humidity RH2i, although this influence is not unequivocal. The most unfavorable conditions regarding nominal temperature effectiveness in terms of frost accumulation are also unclear. In some cases, it was even demonstrated that an increase in the effectiveness can eliminate the ‘frost’ accumulation area.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The original α-model is developed.•The most probable variants of year-round heat exchanger operation are established.•The different frost-free techniques are proposed.
The paper presents an original ...mathematical model based on the modified ε-NTU method, developed for numerical simulations and analysis of coupled heat and mass transfer inside the rotary heat exchanger operating under frosting conditions. The proposed model was validated on the basis of experimental data. Positive validation results indicate that the developed model is capable to predict the behavior of the rotary heat exchanger at high rotor speed. Moreover, the nature of heat and mass transfer processes in the rotary heat exchanger under winter conditions is presented. Different variants of active heat and mass transfer zones in the regenerator channels are revealed. Schemes of safe (frost free operation – without a frost area accumulation) and unsafe operation (with a presence of frost area accumulation) of the device are discussed. Three methods of protecting the heat exchanger against frost are presented and analyzed. The influence of rotor speed on frost formation inside the matrix of rotary heat exchanger is discussed. For analyzed cases the increase in rotor speed by 65% results in an increase in heat exchanger effectiveness (from 0.70 to 0.87) with simultaneous decrease in the size of the frost area accumulation (ΔX¯1) from 0.22 to 0.07. The 68% decrease in the rotor speed was accompanied by a decrease in the heat exchanger effectiveness (up to 0.58) with complete elimination of the frost area accumulation. A similar effect was also observed when the outdoor air temperature slightly increased (from –15 °C to –13.5 °C) while maintaining constant efficiency (0.70) of the heat exchanger.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The mathematical model is useful for analyzing the heat exchanger performance.•The most probable variant of heat exchanger operation is established.•There is a risk of frost accumulation within the ...fixed-bed regenerator.
The paper describes an original mathematical model of heat and mass transfer for a fixed bed regenerator used to determine the effect of internal airflow leakage on heat exchanger efficiency. First, the build and operating principles of a fixed-bed heat exchanger were discussed. The presence of internal leakage was shown along with a proposed solution for its elimination. Then, a mathematical model based on a modified ε-NTU method for numerical simulation and analysis of coupled heat and mass transfer in a fixed-bed heat exchanger was presented. The model was supplemented with an additional detailed submodel designed to determine frost surface temperature and frost growth analysis. Initial numerical simulations of the operation of the model were performed for low outdoor air temperature conditions. Different zones of heat and mass transfer within the regenerator matrix were distinguished. The presence of accumulated frost area and its influence on the course of heat and mass transfer processes was revealed. In addition, the presence of a relative humidity limit curve associated with a mass balance deficiency conditioning the occurrence of frost accumulation was established.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Outdoor air relative humidity affects frost accumulation limit temperatures.•Frost growth does not influence the Number of Transfer Units significantly.•Latent heat flux contributed to the local ...frost density should not be neglected.
This paper tackles the issue of frost formation in non-hygroscopic rotary heat exchangers used for energy recovery from exhaust air under high-speed rotor conditions by means of numerical simulations and experimental approaches. On the basis of some idealized assumptions, a frost growth submodel is presented to predict the behavior of the rotary heat exchanger under frosting conditions. Frost formation is modeled by considering the mass diffusion of water vapor through the frost layer, taking into account supersaturation phenomena. Calculations were carried out using a three-zone model based on the modified ε-NTU method. The local heat transfer coefficient and the NTU calculation method resulting from the influence of the heat exchanger entrance region were also applied. The obtained correlations for the temperature effectiveness agree with the simulation data within uncertainty bounds. The results of the numerical simulations allowed us to determine the outdoor air conditions that initiated the frost accumulation phenomenon inside the thermal wheel for two values of return air relative humidity: RH2i=20% and RH2i=40%. In both cases, the threshold temperature for unsafe operating conditions increases with increasing relative humidity of the outdoor air. Under ‘frost accumulation’ operating conditions, the frost growth rate is approximately five times higher at RH2i=40% than at RH2i=20%. In this regard, the need to implement frost protection techniques increases significantly with an increase in relative humidity of return airflow. Further analysis conducted for the operation of a thermal wheel’s operation under frosting conditions revealed that a latent heat flux contributed to the local frost density should not be neglected in a compact heat exchanger’s model. Interestingly, the operation time of the rotary heat exchanger, and hence the growth of the thickness of the frost layer, has a significant influence on a local heat transfer coefficient α, however, it does not affect the Number of Transfer Units (NTU) visibly.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This paper focuses on the influence of geometrical parameters of the rotary heat exchanger on its operation under high-speed rotor conditions. Proposed mathematical model of heat recovery wheel is ...based on a structure of the counter-flow heat exchanger model. After implementation of a numerical method on the modified α-model, the computer simulations were conducted. They allowed considering the distribution of the active heat and mass transfer zones (“
dry,
” “
wet
” and “
frost
”) on the matrix channel surface depending on the outdoor air conditions for different variants of rotor size. The obtained results indicate that the increase in the wheel’s depth leads to the increase in temperature effectiveness of heat recovery (from 0.667 to 0.814). Moreover, it was also established that the threshold temperature under which the frost is accumulated on the core surface also rises with the rotor depth (from − 10.7 to −9.6 °C). It was concluded that under certain outdoor air temperature conditions (
t
1
i
> –10.7 °C, for rotor depth equal to 0.40 m), the mass transfer rate of the condensed water vapor in the channels of the return air side is equal to the evaporation mass transfer rate in the channels of the supply air side, and hence, the temperature efficiency reaches the same value level as under “
dry
” operating conditions. It was established that only under frost accumulation conditions, the increased temperature effectiveness is observed.
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EMUNI, NUK, OBVAL, SBMB, SBNM, UL, UM, UPUK
The paper presents an analysis of heat and mass transfer processes occurring inside the rotary heat exchanger operating under high-speed rotor conditions for different values of the airflow rate. For ...this purpose the original mathematical α-model was used. Conducted computer simulations allowed to determine the influence of Number of Transfer Units (NTU) of airflow on the temperature effectiveness as well as on the distribution of different active heat and mass transfer zones: “
dry
”, “
wet
” and “
frost
”. It was found that the increase of the values of NTU strictly affects the increase of the effectiveness of heat recovery. Another issue emerging from this study is the fact that in the certain range of low values of NTU there is no “
dry
” area created. It was established that at low values of NTU (NTU≈1) “
frost
” area extremum and sharp drop in the “
frost
” area accumulation are observed.
The paper presents the basic issues related to heating and ventilation of historic sacred buildings based on a review of available literature. It gives guidelines for designing and calculation method ...of useful heat demand as well as shows vulnerable issues which must be considered during designing process. The article focuses on pros and cons of warm air heating and low temperature radiant heating (“friendly heating”). The applicability of using these heating methods in the University Church in Wrocław (Poland) was considered.
The paper presents the course of heat and mass transfer in the fixed-bed heat exchanger. For this purpose, a previously developed original mathematical model of a fixed bed regenerator was used. On ...the basis of the developed model, a computer program was written and verified using experimental data. The validation results confirmed that the developed model can predict the performance of the heat exchanger. The effect of the presence of internal air leaks was analyzed for conditions of sub-zero outdoor air temperatures. Multi-variant numerical simulations carried out under low outdoor air temperature operating conditions made it possible to demonstrate many irregularities. Effectiveness variation was carried out for different thermodynamic air parameters. A maximum increase in device effectiveness (up to 0.92) was demonstrated, confirmed by the additional condensation of water vapor effect and the dominant effect of “wet” area accumulation. In the analyzed cases, a decrease in “frost” area accumulation was observed (by 10%) as the relative humidity of the return air increased. Moreover, the presence of a limiting return air relative humidity (for which the dew point temperature is equal to 0°C) was demonstrated, beyond which the non-accumulated “frost” area was partially transformed to a non-accumulated “wet” zone. A significant change in the effectiveness of the device was noticeable when the relative humidity of the return exceeds 30%, which was also confirmed by the maximum size of the “frost” area for this value (about 49%).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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