Greenhouses are energy-intensive agricultural systems, where the sustainable design of natural ventilation could markedly reduce energy demand while maintaining optimal conditions for plant growth. The performance of natural ventilation arises from a multifaceted interaction among several determinants, including the geometric configuration of the greenhouse, prevailing environmental conditions, and the structural characteristics of ventilation openings and ducts. This study employed computational fluid dynamics (CFD) to assess the influence of roof inlet design on airflow distribution, regulation of canopy temperature, and energy performance in a single-span greenhouse measuring 20 × 10 × 6 meters. Six ventilation configurations were evaluated by varying the quantity and shape of roof inlets: three large inlets and ten smaller inlets, each with rectangular, oval, or circular geometries. The plant canopy was modeled as a porous medium to realistically capture aerodynamic resistance. Mesh independence was validated using outlet mass flux, and simulations were conducted under steady-state natural ventilation conditions. Key performance indicators included airflow velocity, temperature distribution, ventilation rate, wall shear stress (WSS), air changes per hour (ACH), and estimated annual energy saving. Results of the analysis revealed that circular and oval inlets enhanced air mixing and reduced thermal gradients within the canopy, whereas rectangular inlets generated localized recirculation zones and elevated WSS, resulting in lower energy efficiency. The inlet geometry and quantity played a critical role in the sustainable design of greenhouse ventilation. By integrating CFD-based airflow analysis with energy-saving assessments, this study offered a practical framework to guide greenhouse operators in optimizing ventilation strategies that balance productivity, thermal comfort, and long-term energy sustainability.
This paper discussed the possibilities of using the developed Dignet Energy Platform (DEP) for modeling and optimization of bioenergy production. The DEP presents a set of software tools based on a mathematical model to calculate the desired output and the profitability of investments in a renewable energy source based on input parameters. By using Multi-Criteria Decision Analysis (MCDA), the DEP selects an optimal variant of energy or fuel production from biomass. This tool enables the simplification of complex and biomass energy production-related calculations while facilitating the customization of each individual element in the bioenergy production process. The user could use a simple procedure to “simulate” the production parameters and choose the best option from a set of biomass-based projects. Criteria describing the various projects were selected by the users and calculated by the DEP. These criteria helped select the appropriate optimal project by multi-criteria optimization. In this paper, several chains of biomass fuel/heat/electricity production applicable to the settings in the Republic of Croatia and the region were analyzed. Results in this research provided selection of optimal chains for the production of solid fuels and energy, including heat and combined heat and power (CHP) from different categories of biomass. The DEP is proved to be a practical and effective tool in selecting the optimal project of biomass energy production.
This study investigates the design and performance of altitude test benches for piston engines with power outputs up to 200 kW. The primary objective is to generate controlled depressions within an enclosed engine bay to reproduce atmospheric conditions corresponding to altitudes ranging from sea level to 14,000 m. Three configurations are examined: an ejector–diffuser system derived from National Advisory Committee for Aeronautics (NACA) principles, a Venturi device powered by an auxiliary diesel engine (Cursor 13), and a centrifugal turbocharger (Holset HY55V) mechanically coupled to the same auxiliary engine. Computational Fluid Dynamics (CFD) simulations are performed to evaluate the pressure and velocity distributions within the test chamber and its associated flow components. The ejector-diffuser arrangement achieves a moderate pressure reduction but exhibits flow separation in the diffuser at large expansion angles, limiting its efficiency. The Venturi system achieves a greater vacuum level, reducing the chamber pressure to approximately 76 kPa, equivalent to an altitude of around 2,500 m. The turbocharger-based configuration demonstrates the highest performance, achieving a chamber pressure of approximately 15 kPa—equivalent to an altitude of 14,000 m—through appropriate adjustment of compressor rotational speed and intake valve opening. This configuration also ensures a faster transient response and enhanced stability of airflow and pressure distribution. The results highlight the importance of proper integration between auxiliary propulsion systems, component sizing, and boundary condition definition to achieve accurate altitude simulation. The proposed approach demonstrates that combining a variable-speed compressor with active flow control enables flexible reproduction of both steady-state and transient operating conditions. The findings provide practical guidelines for developing cost-effective, reliable, and versatile altitude test benches suitable for experimental evaluation and calibration of high-power piston engines under simulated high-altitude environments.
The combustion behavior of blended petroleum–biofuel mixtures has increasingly been investigated as interest grows in low-toxicity, biodegradable, and energy-dense biomass-derived fuels. Among higher alkanols, n-butanol is recognized for its favorable physicochemical properties and its compatibility with gasoline-range hydrocarbons (HC) such as iso-octane. In this context, a systematic evaluation of laminar flame propagation and instability characteristics is essential for understanding the combustion performance and operational safety of blended fuels. In the present study, the laminar burning velocity (LBV) and cellular instability of premixed iso-octane/n-butanol/air flames were quantified for a wide range of equivalence ratios (0.7–1.5) at an initial temperature of 423 K and ambient pressure. It was observed that the LBV increased consistently with the addition of n-butanol, whereas the Markstein length (Lb) decreased. Analysis of cellular structures revealed that diffusive-thermal instability strengthened monotonically as the equivalence ratio increased, resulting in more unstable flame propagation under fuel-rich conditions. In contrast, the hydrodynamic instability exhibited a non-monotonic trend, first intensifying and subsequently diminishing with increasing equivalence ratio. The critical Peclet number decreased continuously across the equivalence-ratio range, while the critical flame radius varied non-monotonically. The incorporation of n-butanol was found to enhance both diffusive-thermal and hydrodynamic instabilities and to reduce the critical Peclet number and critical flame radius. These findings underscore the need for careful control of combustion stability in practical applications involving iso-octane/n-butanol mixtures and provide fundamental insight into the flame-structure evolution associated with next-generation alternative fuels.
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