iam monitoring pressure in pipe.(total pressure)it decreases and goes to negative value and then increases.does the pressure goes to negative value???

If you are looking for useful feedback, start by describing your problem properly. Is it a short pipe or long one? Is it heated? Isothermal? Is it constant area? Is it.....etc What are your underrelaxation values?.......etc log on www.studentfluent.com and read the tutorial about pipe analysis

the pipe is 10m length and 1 inch dia.there is no heating .the mixture of two fluids(oil and kerosene) enters at 1 atm pressure. the urf are momentum 0.7 and volume fraction 0.9 rest all default.

The only difference between the tutorial and your case is that you are using two fluids. Have you defined your fluid zone properly? What fluid model have you used? Multiphase mixture model or Multiphase VOF or have you used the species transport model? You see, unphysical results, especially when it comes to the calculation of pressure is a sign that pressure-velocity coupling and calculations are not set properly, so check your flow model. If you are solving the correct model for your case, then reduce the Under relaxation parameters. You are doing well observing your results, but observations have to be followed by good and correct analysis, so one can feel comfortable with the results especially when they are obtained by a numerical scheme. Good Luck