Fast flame speeds and rates of pressure rise in the initial period of gas explosions in large L/D cylindrical enclosures

Flame speeds and rates of pressure rise for gaseous explosions in a 76 mm diameter closed cylindrical vessel of large length to diameter ratio (L/D = 21.6), were quantitatively investigated. Methane, propane, ethylene and hydrogen mixtures with air were studied across their respective flammability ranges. Ignition was affected at one end of the vessel. Very fast flame speeds corresponding to high rates of pressure rise were measured in the initial 5–10% of the total explosion time. During this period 20–35% of the maximum explosion pressure was produced, and over half of the flame propagation distance was completed. Previous work has concentrated on the later stages of this type of explosion; the development of tulip flames, pressure wave effects and transition to turbulence. The initial fast phase is very important and should dominate considerations in pressure relief vent design for vessels of large L/D.     

Soil erosion in developing countries: a socio-economic appraisal

Soil erosion is the single most important environmental degradation problem in the developing world. Despite the plethora of literature that exists on the incidence, causes and impacts of soil erosion, a concrete understanding of this complex problem is lacking. This paper examines the soil erosion problem in developing countries in order to understand the complex inter-relationships between population pressure, poverty and environmental-institutional dynamics. Two recent theoretical developments, namely Boserup's theory on population pressure, poverty and soil erosion and Lopez's theory on environmental and institutional dynamics have been reviewed. The analysis reveals that negative impacts of technical change, inappropriate government policies and poor institutions are largely responsible for the continued soil erosion in developing countries. On the other hand, potential for market-based approaches to mitigate the problem is also low due to the negative externalities involved. A deeper appreciation of institutional and environmental dynamics and policy reforms to strengthen weak institutions may help mitigate the problem.     

The water footprint of hydroelectricity: a methodological comparison from a case study in New Zealand

Hydroelectricity has been rated to have a large water footprint (WF) on global average. We assessed the WF of hydroelectricity by three different methods using New Zealand as a case study. The first (WF-1) and second (WF-2) methods only consider the consumptive water use of the hydroelectricity generation system, while our third method (WF-3) accounts for the net water balance. Irrespective of the method, the WF of New Zealand’s hydroelectricity was found much smaller than the commonly cited international value of 22 m³ GJ⁻¹. Depending on the method, the national WF ranged from 1.55 m³ GJ⁻¹ (WF-3) to 6.05 m³ GJ⁻¹ (WF-1). The WF- 3 considers the net water balance including rainfall, which is the key driver for replenishing water resources. It provides meaningful information that helps our understanding of the differences of the WF in locations, which are diverse in terms of water resource availability. We highlight the effects of local climatic differences and the structural specifics of a hydroelectricity scheme on the WF. The large variation in the WF of hydropower across New Zealand illustrates the inappropriateness of using global average values. Local values, calculated using our hydrologically rational method, must be used.