It is attempted in the present investigation to study the influence of internals in the riser of liquid-solid fluidized bed. Experiments on bed expansion characteristics in liquid-solid fluidized bed have been carried out in a conventional liquid-solid fluidized bed and in a fluidized bed having string plates of spheres and twisted tapes as internals covering wide range in operating conditions. It has been observed in the present investigation that an increase in particle diameter, density and twist ratio is found to decrease the bed voidage and an increase in the pitch of the string increases the bed voidage. Further it has been observed that the bed voidage is higher in fluidized bed without internals when compared to fluidized bed with internals for a given operating conditions. A model based on slip velocity has been developed for predicting bed voidage and observed that more than 92% of the experimental data match with the model predictions within 10% error.