An Application of Nonlinear Autoregressive(NARX)Model to Predict Adsorbent Bed Temperature of Solar Adsorption Refrigeration System

In any solar adsorption refrigeration system,there are three major components:a solar collector adsorbent bed,a condenser and an evaporator.All of those components operate at different temperature levels.A solar collector with a tubular adsorbent configuration is proposed and numerically investigated.In this study,a nonlinear auto-regressive model with exogenous input is applied for the prediction of adsorbent bed temperature during the heating and desorption period.The developed neuronal model uses the MATLAB Network toolbox to obtain a better configuration network,applying multilayer feed-forward,the TANSIG transfer function,and the back-propagation learning algorithm.The input parameters are ambient temperature and the uncontrolled natural factor of solar radiation.The output network contains a variable representing the adsorbent bed temperature.The values obtained from the network model were compared with the experimental data,and the prediction performance of the network model was examined using various performance parameters.The mean square error(MSE)and the statistical coefficient of determination(R2)values are excellent numerical criteria for evaluating the performance of a prediction tool.A well-trained neural network model produces small MSE and higher R2 values.In the current study,the adsorbent bed temperature results obtained from a neural network with a two neuron in hidden layer and the number of the tapped time-delays d=9 provided a reasonable degree of accuracy:MSE=1.0121 and R2=0.99864 and the index of agreement was 0.9988.This network model,based on a high-performance algorithm,provided reliable and high-precision results concerning the predictable temperature of the adsorbent bed.

The Impact of Bed Temperature on Heat Transfer Characteristic between Fluidized Bed and Vertical Rifled Tubes

In the present work,the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes(38mm-O.D.) in a commercial circulating fluidized bed(CFB) boiler.Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246 mm.The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler.A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach.The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.

Effect of Scale-up on Heat Transfer Characteristics in Upper Splash Region of Circulating Fluidized Bed Risers

In the present work, steady state heat transfer experiments were conducted in the upper splash region of three cold circulating fluidized beds （CFB）, B 1, B2 and B3, with height of each 2.85 m and bed cross sections of 0.15 m × 0.15 m, 0.20 m × 0.20 m and 0.25 m × 0.25 m, respectively. Experiments were conducted under similar operating conditions on all the three CFt3 setups for two different non-dimensional air velocities （U＊ = 5 and 8） and two different sand inventories with average particles size of 460 μm. Effect of cross section of riser on heat transfer characteristics was studied. Bed temperature distribution across the heater placed in the upper splash region of riser was measured at two sections, at a height of 1.96 m and 2.24 m above the distributor plate. Axial distribution of local heat transfer coefficients along the height of heater were evaluated and compared for different bed cross sections. Results obtained were compared with the available literatures.

Restart-up Performance of a CFB Boiler after a Sudden Power Failure Accident

Due to the massive bed materials in its typical main loop,a circulating fluidized bed(CFB)boiler may face greater trouble in operation and restart-up if an unplanned sudden power failure happens in the power plant.This study aims to explore the restart-up performance of a 170 t/h natural circulation CFB boiler after a sudden power failure and evaluate whether the hot restart-up can be realized.The heat transfer process after the restoration of the primary air is modeled.Validating the isothermal property of the bed after the restoration of the power and air supply,the correlation among the accident duration time,bed temperature after the power restores,inlet air temperature,air supply time,and air velocity is proposed.The predicted results indicate that the major influencing factor of the bed temperature during this process is primary air velocity.To provide guidelines for judgment on whether the hot restart-up can be realized,the maximum values of the air supply time and accident duration time for hot restart-up for different types of coals are given.The results show both of them have a rapid decrease as the coal ignition temperature in a CFB boiler increases from 450°C to 650°C,which also means that the coal ignition temperature plays a very important role in the restart-up process.Based on the simulation results of bed temperature during the accident,it is also estimated that the drum level drops by 77.7 mm after 8 hours but still stays in the permissible range.