Search

Thermoacoustic devices represent a significant future opportunity in the fields of energy generation and refrigeration. A key component of this type of device is the regenerator, where the conversion between acoustic energy and thermal energy takes place. This conversion occurs due to an externally imposed temperature gradient on the wall of the regenerator channels. Hence, this study concerns the physics of sound waves in the proximity of such walls. It establishes a new analytical framework which clarifies the disturbance energy conservation in thermoacoustic devices. In this framework, a thermoacoustic production term is proposed to quantify the generation or consumption of disturbance energy originating from the temperature gradient. An extended disturbance energy flux term is identified to account for wave growth or decay through the regenerator. The disturbance energy balance relation states that the disturbance energy flux equals the thermoacoustic production less the viscous and thermal dissipation resulting from gradients of fluctuating velocity and temperature. The analytical framework is implemented into an axisymmetric cylindrical domain; the two-dimensional nature of this work helps to uncover that the wave always decays in the region close to the wall. A dimensional analysis is conducted to identify the controlling parameters, namely the Womersley, Helmholtz and Prandtl numbers. A parametric study of the Womersley and Helmholtz numbers is conducted to showcase the new analytical methodology; the results make it possible to optimize the geometry, wave properties and working conditions of a thermoacoustic device according to the preference of its efficiency, loss and output. This study then validates the analysis in an actual thermoacoustic engine environment. A straight waveguide with a loudspeaker at one end and wave absorbing material at the other simulates a travelling-wave condition of various frequencies. A novel regenerator consisting of 671 thin stainless-steel tubes were inserted into two copper heat exchangers to form an integral block. The design enables reproduction of the geometry and a known, constant, precisely controlled wall temperature gradient as in the model. The selection of material also encourages radial heat conduction while minimising the axial one. The experimental results show excellent agreement with the numeral model and validates the analytical framework proposed. The maximum measured wave growth was 1.76 when the temperature difference between hot and cold end of the regenerator was 303K.

ModClark and Clark’s Unit Hydrograph (Clark’s UH) within HEC-HMS software are distributed and lumped models, respectively. Clark’s UH simulates the transformation and attenuation of excess precipitation, and requires time of concentration (Tc) and Storage Coefficient (R) parameters. ModClark transformation accounts for variations in travel time to catchment outlet from all regions of a catchment, and it additionally requires gridded representation of a catchment and Gridded cell-based input files. Four cases (three from observed rain, and one from radar reflectivity) of three chosen events were specifically chosen and examined for the comparison of simulation results with the same estimated initial parameters apart from different rainfall inputs. The Upper Waimakariri River Catchment was divided into ten subcatchments, and the HEC-HMS basin model parameters were estimated by using the physical/hydrological characteristics. However, ModClark transformation was unavailable because of an output error from converting ASCII to gridded Soil Conservation Service Curve Number (SCS CN) format by the conversion tool – ai2dssgrid.exe. Therefore, Mean Aerial Precipitation (MAP) for each subcatchment was calculated by Thiessen polygon method combined with an overlay analysis for grid-cell-based rainfall estimation from radar with geographic information system (GIS) tools. The automated calibration/optimisation procedure included in HEC-HMS package was applied to the cases which showed a deviation between simulation and observed flows. The purpose is to ‘optimise’ the initial estimates of parameters only in a mathematical-fit manner based on the observed flows from the only discharge gauge at Old Highway Bridge (OHB). The TC values calculated from the five equations vary in a relatively narrow range apart from the one from Bransby-Williams equation. Therefore, the values from all the other four equations were averaged and used as the initial TC input. The simulation results showed that there was a notable difference between observed and simulated hydrographs for some case studies even though TC, R, CN, and lag time were calibrated/optimised separately. Also, radar estimated rainfall and grid-based data storage system (DSS) need more investigations.