exhaust hood

An air curtain generated by a jet is used to enhance an exhaust hood’s capture ability in many research studies on local ventilation systems. This paper focuses on experimental methods to investigate the flow characteristics formed by an exhaust hood associated with a jet. The basic flow characteristics of this kind of exhaust hood are obtained by smoke visualization as a jet forms an air curtain, and the flow field is a combination of three parts: the jet flow region, the exhaust flow region and the vortex flow region.

All over the world, Chinese restaurants can be found everywhere and the Chinese foods are famous.The Chinese food preparation procedure includes: frying, stir-frying, stew, etc. In the process of cooking,It releases large amounts of aerosol which is the mixture of vapor, PAHs (Polycyclic AromaticHydrocarbon), VOCs (Volatile Organic Compounds), etc. When the aerosol mixes with combustionexhaust-gas, the mixture becomes main air contaminant in kitchen.

The performance of an exterior hood is known to be affected by the cross draft (1, 2). Based on the knowledge from a classical "Rankin's nose" or "semi-infinite body" problem, in which the opening shrinks to a sink instead of a finite opening, the exhausted airflow combining the cross draft forms a capture envelope in front of the hood(3). All streamlines within the envelope lead to the hood opening, those outside of the envelope lead to infinity. Therefore, contaminant released inside the envelope tends to be captured by the hood; otherwise, it tends to escape beyond capture.

Exhaust hoods are used in many industries to remove contaminant from a region close to the source( s) of the generation by the withdrawal of air and contaminant. In comparison with traditional exhaust hoods, the Aaberg exhaust system, with its additional jet, can significantly improve the capture efficiency of the hood. Since the 1980's experimental investigations and mathematical analyses on the Aaberg exhaust systems have been performed by Hogsted (1), Hyldgard (2), Pedersen and Nielsen (3) Fletcher and Saunders (4) and Hollis (5).

In 1998, a program was initiated to develop an innovative backshelf hood system that could achieve a much lower capture and containment (C&C) exhaust rate than traditional backshelf hoods. As part of this effort, an evaluation of the state-of-the-art tools in use in commercial kitchen ventilation in the United States was undertaken. This paper presents the new hood concept KVL, a description of the latest techniques available for determining C&C performance, and comparisons of the KVL new hood concept to other hoods.

Standard design methods for local exhaust hood design require the selection of the necessary capture velocity and then application of empirical equations relating capture velocity with hood flow rate. The selection of capture velocity depends on hood geometry, source generation rate, and disturbances in the vicinity of the local exhaust hood. Current design techniques for vapor degreasers require a hood flow rate of 0.25m3s-1 per m2 of tank area.(1) The design method does not account quantitatively for crossdrafts, but instead recommends eliminating crossdrafts.

The purpose of the work described in this paper is to develop a mathematical model of downdraft exhaust hoods in order that ways of improving these hoods efficiency can be examined. In this initial study the model developed is twodimensional. The flow has been assumed to be ideal and the complex potential considered. By use of conformal mappings the airflow in the vicinity of a bench, which is extracting air and also has air being blown down from above, is modelled. Various ratios of extraction to downdraft are considered in order to investigate the most efficient method of operation.

The efficiency of a kitchen ventilation system is usually determined by its ability in heat and effluent removal. The main part of a ventilation system is the hood, with its face (or capture) velocity. Heat generation associated with the cooking process is the main factor that affects the thermal comfort. The heat removal capability is studied under different capture velocities so as to determine the minimum requirement for efficient removal of heat and effluent.