Fuel injection System

Nozzle Selection

Spray Nozzles

A nozzle is a mechanical device that is designed to control fluid flow characteristics as fluid, either liquid or gas, exits from an enclosed chamber into a chosen medium.  Nozzles are relatively simple in design, usually a pipe or tube that varies in its cross-sectional area from one end to another.  Nozzles are used to control different characteristics of fluid flow such as rate of flow, direction, speed, mass, etc.  Nozzles also have the ability to control exit droplet size and fluid spray pattern, which are the pertinent characteristics in this project.

Nozzle Selection

The following characteristics are considered when selecting a nozzle: spray pattern, capacity, spray angle, and droplet size. Spray pattern is dependent on the function of the nozzle for its specific application. For the fuel jet injection system a uniformly distributed spray pattern with a possible Gaussian distribution is needed. Moreover, a relatively wide range of droplet sizes, ranging from 500µm to 5000µm, need to be generated. Full cone spray nozzles with different capacities (12-0.3 gpm) under different pressures (10-100psi) can produce droplet sizes in the desired range.

Below a tabulated summary is provided that contains different fluid properties and nozzle characteristics and their effect on one another: 

For example; If  there is an increase in the viscosity of the fluid the associated  "Nozzle Characteristic" is affected as indicated by the indicated amount....

 Table 1:  (Spraying Systems Co., [1])

There are many terminologies used to describe the drop sizes within a spray; three of the most commonly used terminologies are:

• Volume Median Diameter (VMD) also expressed as D_v0.5 and Mass Median Diameter (MMD)

Drop size is expressed in terms of the volume of liquid sprayed, and when measured in terms of volume is a value with 50% of total volume of liquid sprayed contains drops with diameters larger than median value and 50% with smaller diameters.

• Sauter Mean Diameter (SMD) also shown as D₃₂

Fineness of spray is expressed in terms of surface area produced by the spray. SMD is the diameter of a drop with the same volume-to-surface area ratio as the total volume of all the drops to the total surface area of all the drops

• Number Median Diameter (NMD) or D_{No. 5}

Drop size is stated as the number of drops in the spray. So, 50% of the drops by count or number are smaller than the median diameter and 50% are larger than the median diameter.

Volume Median Diameter or VMD is the most popular known expression used to represent drop size distribution in most nozzles.

Final selection of the nozzle is done by examining a graph provided by the manufacturer of the nozzle that gives spray particle size in VMD versus pressure for standard full cone nozzles. The graph is done for many nozzles with different capacities. It is very important to mention that these tests were done for water and the particle size provided is for water at room temperature. The graph is only used to aid us through selecting the correct nozzle, but is in no case the final nozzle characterization. The actual characterization of the nozzle will be done by us.

   "Nozzle Selection, graph spray particle versus pressure (Spraying Systems Co,[2])"

The actual nozzles selected from Spraying Systems Company are the following:

Table 2- nozzle selection

 REFERENCES